21z Forecast Discussion for 9-30-10

Synopsis…

Tropical moisture continues to roll up the eastern seaboard, causing some impressive rainfall totals and hazardous travel conditions. Western PA is already cool and dry, and Eastern PA will continue to be inundated through the evening, but Central PA is the boundary zone. Farther east, the rain will continue through Friday morning. A cold front will work its way slowly through the area overnight and Friday, leading to some of the coolest temperatures the state has seen in quite a while. Other than isolated sprinkles this weekend, we will finally be able to dry out over the next week.

 

Short Term…

As previous shift aptly named it, this dual-wave system continues to move through the mid-Atlantic. Any rain in the west is shutting off now, and even KUNV's heaviest precip is over as of 21z. But, flood watch continues for estrn PA thru Friday aftn due to excessive rainfall topping 7" in spots. New 18z WRF run seems to have a good handle on the timing and placement of the precip shield across PA, and it suggests a secondary wave pushing through the SE and NE zones from about 12z-18z Friday afternoon, causing some concern with our "rain tapering off" forecast. We do expect areas west of about I-81 to stay dry tomorrow, and can definitely see clearing after FROPA, so we kept these areas mainly dry. Tight pressure gradient should lead to breezy conditions Friday.

 

Long Term…

Cold air returns! The 540-line makes its first appearance in a long time and dips well south of Erie for early next week. Rural areas of wstrn PA will see lows in the 30s, with cold spots (BFD?) approaching freezing. WRF has some odd-looking precip output Saturday associated with leading edge of the coldest air. We are not sure if there is enough instability for shwrs, so we kept most areas dry and mentioned a slt chc of a sprinkle for the weekend. Much-below-avg temps look to continue through the week, with highs stuck in the 50s in many areas as a strong trough dominates the pattern.

 

Forecaster: Matt Mahalik

Technical Discussion

Synopsis...

 

Large duel-wave system, including the remnants of Nicole, moving up the east coast on Thursday will send copious amounts of moisture and precipitation across almost the entirety of the Mid-Atlantic region.  Low pressure system and associated frontal boundary will move out during the day on Friday and a trough sets up shop for the weekend.

 

Short term (18z Thursday - 12z Friday)...

 

A slight difference from the previous forecast is that most of the models, including the SREF ensemble data, show a slight trend westward on the tracks of the low pressure systems moving up the coast.  The GFS, NAM, ECMWF, and the ensembles all show the remnants of Nicole going just east of the spine of the Appalachains, bringing essentially a perfect setup for heavy rainfall across central-eastern Pennsylvania.  State College has seen around .40" so far with reports out of Lancaster of over 2" within a 3-hour period between 6 and 12z.  Trickiest part of the forecast is pinpointing the tight gradient in precipitation over western-central portions of the state.  Am currently thinking that heavier amounts can extend as far west as a Johnstown - Clearfield and northward line (with amounts of up to 3-4" possible).  PWATs across the state are well over an inch given the strong conveyor belt of tropical moisture riding up the stalled frontal boundary over the eastern US.  As for eastern sections of the state, the SREF total precipitation amounts have a whopping 6-8"+ mean rainfall range over a line from Harrisburg - Scranton from about 12z Thursday - 12z Friday, quite unkown territory for the ensemble means.  I expect this to verify given current strong convergence zone over northern virginia and eastern Maryland on current observation sites.  It still looks like far northwestern sections of the state will see mostly very light precipitation from this event, but the 6z WRF has heavy precipitation just to the east of Erie and Meadville, whereas the GFS and SREF have heaviest precipitation farther east. Flash flooding will likely be a widespread problem across the Susquehanna Valley over the next day or so. Temps will be around 60 in the far northwestern sections of the state and in the mid-upper 70's in the southeast near Philadelphia for highs today - that may be the simplest portion of the forecast. 

 

Long Term (after 12z Friday)...

 

As for the longer term, after the front and Nicole move out, a trough establishes itself over the northeast later Friday and into the weekend cooling temps down quite significantly into the upper 50's and lower 60's for highs on Saturday and Sunday across much of the state.  WRF and GFS hint at some type of light precipitation on Sunday between 0z and 18z across much of the state but this should not be significant.  We'll need a break after the rainfall the state gets in the next 24 hours...and the entire Mid-Atlantic for that matter.

Forecaster: Kyle Imhoff

Technical Discussion

Synopsis...

 

Large duel-wave system, including the remnants of Nicole, moving up the east coast on Thursday will send copious amounts of moisture and precipitation across almost the entirety of the Mid-Atlantic region.  Low pressure system and associated frontal boundary will move out during the day on Friday and a trough sets up shop for the weekend.

 

Short term (18z Thursday - 12z Friday)...

 

A slight difference from the previous forecast is that most of the models, including the SREF ensemble data, show a slight trend westward on the tracks of the low pressure systems moving up the coast.  The GFS, NAM, ECMWF, and the ensembles all show the remnants of Nicole going just east of the spine of the Appalachains, bringing essentially a perfect setup for heavy rainfall across central-eastern Pennsylvania.  State College has seen around .40" so far with reports out of Lancaster of over 2" within a 3-hour period between 6 and 12z.  Trickiest part of the forecast is pinpointing the tight gradient in precipitation over western-central portions of the state.  Am currently thinking that heavier amounts can extend as far west as a Johnstown - Clearfield and northward line (with amounts of up to 3-4" possible).  PWATs across the state are well over an inch given the strong conveyor belt of tropical moisture riding up the stalled frontal boundary over the eastern US.  As for eastern sections of the state, the SREF total precipitation amounts have a whopping 6-8"+ mean rainfall range over a line from Harrisburg - Scranton from about 12z Thursday - 12z Friday, quite unkown territory for the ensemble means.  I expect this to verify given current strong convergence zone over northern virginia and eastern Maryland on current observation sites.  It still looks like far northwestern sections of the state will see mostly very light precipitation from this event, but the 6z WRF has heavy precipitation just to the east of Erie and Meadville, whereas the GFS and SREF have heaviest precipitation farther east. Flash flooding will likely be a widespread problem across the Susquehanna Valley over the next day or so. Temps will be around 60 in the far northwestern sections of the state and in the mid-upper 70's in the southeast near Philadelphia for highs today - that may be the simplest portion of the forecast. 

 

Long Term (after 12z Friday)...

 

As for the longer term, after the front and Nicole move out, a trough establishes itself over the northeast later Friday and into the weekend cooling temps down quite significantly into the upper 50's and lower 60's for highs on Saturday and Sunday across much of the state.  WRF and GFS hint at some type of light precipitation on Sunday between 0z and 18z across much of the state but this should not be significant.  We'll need a break after the rainfall the state gets in the next 24 hours...and the entire Mid-Atlantic for that matter.

Forecast Discussion

Synopsis...

A huge rainmaker is coming our way... the remnant low of Nicole will be moving up the coast on Thursday, bringing very heavy rainfall to the central and eastern parts of the state. Many areas will receives 2 or more inches of rain tomorrow, with some spots potentially seeing 8+ inches of rainfall. A weak disturbance could bring a scattered shower to the northwest Friday night into Saturday morning. Chilly lows will also be a story, some lows dropping below 40 degrees over the weekend.

Short Term...

The biggest story of this forecast period is the remnant low of TS Nicole moving up the coast. The NHC currently has it classified as a gale center, so areas in the extreme eastern parts of the state will see some very breezy conditions in lieu of the heavy rain... Rain totals from this storm could potentially be greater than 8 inches locally, with pretty much everywhere seeing at least 2-4 inches of rain. Areas in the west, such as Pittsburgh, should be spared all together of this system, probably getting mostly cloudy skies. While temperatures will be seasonable on Thursday, they will drop significantly on Friday. The average forecast MOS prediction is about 10 degrees cooler from region to region. This trend of cooler temperatures and much cooler overnight lows will continue into the weekend. Friday looks to be a mostly clear day across the state, with a NW flow and evaporational cooling keeping the air stable and mostly cloud free.

Long Term...

A weak trough looks to sag into the NW region late night Friday into Saturday morning. Most regions will see a 30-40% chance of a shower on Saturday. This trough though, according to GFS and WRF outputs, diminishes by the evening on Saturday, so expect mostly cloudy skies overnight Saturday. A similar situation sets up on Sunday with another trough building in from the NW, though this one won't be as potent. Some areas will see a scattered shower out of this, but shouldn't be anything significant. Regardless, the NW flow stimulated by both of these systems will keep temperatures well below average through the weekend. A westerly flow begins to build in on Monday, which will warm temperatures back up. Also, for Monday, a dry air mass also looks to build in. Next week looks to start off beautifully.


Andrew Dzambo


We Are... Penn State Meteorology

                                                                Technical Discussion

Synopsis

 A trough in the south will bring moisture associated with tropical storm Nicole up toward the state for late Wednesday night into the day on Thursday. The models are still a little off for the placement of this low pressure coming up the coast. Heavy rain is associated with this system and the mixture of rain and clouds will keep temperatures down for Thursday. A weak cold front will push the disturbance out of the way but a trough will linger into the weekend. This will also bring a shot of cooler air in the region and also keep the chance for a lingering shower to occur.

Short-term(Thursday-Friday)

The moisture from tropical storm Nicole will arrive overnight and last into the day on Thursday. The models are still unclear of the exact track of the system. The 12z run of the NAM has the storm more towards the east, bringing the heaviest rains towards more of the eastern parts of PA. The GFS has the storm more inland over central PA, with the heaviest rains over central and eastern PA.  Im leaning towards the NAM, but I kept the chance for heavy rain across the state. Wherever the placement the rainfall totals will be great with GFS totals up to 3-4 inches over some parts of the state. This means the possibility for small stream and street flooding is there. A weak cold front will pass through and push out the tropical system for the day on Friday. Cool air will move in behind this front bring below average temperatures to the region.

Long-term(Saturday-Sunday)

A trough will build in and stick around for the weekend. This will also filter even cooler air to the region, as temperatures this weekend will have a hard time getting out of the 50s. The GFS shows the chance for an isolated shower for both days. I put it in for the day on Sunday as the greatest divergence will be over us that day which could produce a few showers. Otherwise overnight temperatures will be very chilly as some of the cooler, northern spots could even get down to freezing.  These northern spots could easily see frost especially on Sunday night, with low temperatures dipping down close to freezing.

                                                                Technical Discussion

Synopsis

 A trough in the south will bring moisture associated with tropical storm Nicole up toward the state for late Wednesday night into the day on Thursday. The models are still a little off for the placement of this low pressure coming up the coast. Heavy rain is associated with this system and the mixture of rain and clouds will keep temperatures down for Thursday. A weak cold front will push the disturbance out of the way but a trough will linger into the weekend. This will also bring a shot of cooler air in the region and also keep the chance for a lingering shower to occur.

Short-term(Thursday-Friday)

The moisture from tropical storm Nicole will arrive overnight and last into the day on Thursday. The models are still unclear of the exact track of the system. The 12z run of the NAM has the storm more towards the east, bringing the heaviest rains towards more of the eastern parts of PA. The GFS has the storm more inland over central PA, with the heaviest rains over central and eastern PA.  Im leaning towards the NAM, but I kept the chance for heavy rain across the state. Wherever the placement the rainfall totals will be great with GFS totals up to 3-4 inches over some parts of the state. This means the possibility for small stream and street flooding is there. A weak cold front will pass through and push out the tropical system for the day on Friday. Cool air will move in behind this front bring below average temperatures to the region.

Long-term(Saturday-Sunday)

A trough will build in and stick around for the weekend. This will also filter even cooler air to the region, as temperatures this weekend will have a hard time getting out of the 50s. The GFS shows the chance for an isolated shower for both days. I put it in for the day on Sunday as the greatest divergence will be over us that day which could produce a few showers. Otherwise overnight temperatures will be very chilly as some of the cooler, northern spots could even get down to freezing.  These northern spots could easily see frost especially on Sunday night, with low temperatures dipping down close to freezing.

Synopsis:

Wednesday will be rather calm with a mix of clouds and sun. Tropical depression 16 (possibly to become Nicole), is currently crossing Cuba, will interact with an east coast trough to bring a drenching rain throughout much of the commonwealth on Thursday. After this clears out early Friday, a strong trough will bring about the coolest weather of the season thus far.  Clouds will decrease by the end of the day Friday.  Partly cloudy skies will be seen this weekend with temperatures well below average.

Short term (Wed/Wed night):

A weak high pressure will be over the state during the day Wednesday with calm winds.  There will be a mix of clouds and sun.  This will be short live though as a vort max shifts southward combining with the tropical disturbance near Cuba and Florida.  This system will push northward bringing with it copious amounts of moisture.  Most areas will be overcast by late evening as high RH move into the 700 & 850 levels. 

Long Term (Thursday – Sat)

As the extra tropical system pushes northward, a deep trough is clearly visible over the eastern half of the country.  This trough is responsible for the unseasonably cold weather that will be seen this weekend.  Both the WRF and GFS show precipitation beginning between  2-8 am on Thursday.  As of now, it seems that most ensemble members project for the center of the Low to go right across the center of Pennsylvania near Centre County.  The SREF plumes show that most regions (minus the NW) have mean /median ensemble rain amounts of around three inches.  The min ensemble members in most areas are above 0.5.  Some members say up to 5-6 inches by the end of the storm.  I'd say that many areas will likely see at least two inches of rain.  Exact amounts will depend on track of storm.   Rain will taper off early Friday morning.  Partial clearing will occur Friday afternoon as dry air moves into place, along with chilly temperatures.  Continued NW flow will continue CAA into the region through the weekend.  The GFS pushes the 540 line well below the state into Northern Virginia.  Partly clouds skies will be seen over the eastern portion of the state this weekend.  Areas in the west could see more cloudiness due to the flow over the lake.  Some areas could experience lake effect showers. 

Dan the man the weather man dan Pollak

Technical Discussion

Synopsis...

Rain tapering off from W to E today as low pressure moves into SE Canada.  Pleasant tomorrow but clouds will remain (esp. North).  Next system approaches from south on Wed night and could bring torrential rain to eastern half of PA on Thurs/Thurs night.  Expect major cool down by the weekend with temperatures well below normal.

Short Term (Tues-Wed)...

Low pressure over Erie, PA will continue to move away from the region today, dragging a cold front through the entire state by the evening.  Most of the western half of the state has seen the last of the rain today but a few lingering showers are possible.  Farther east, scattered showers and storms are likely (esp in NE/SE) until this evening.  SPC has E third of state in SLT risk for severe weather today (damaging winds main threat).  This threat should be over by evening.  Entire state dries out tonight but clouds should remain (esp in NC/NW as usual).  Temperatures tonight will not be too chilly due to the cloud cover and lack of a chilly airmass behind this system.  Expect a mix of sun and clouds tomorrow as we are in between systems.  High temperatures should be near normal.

Long Term (Wed Night-Sat)...

The fun begins on Wed night as a large plume of tropical moisture associated with what could become Tropical Storm Nicole pushes northward along the Eastern Seaboard.  This system will interact with another short wave dropping down from the Upper Midwest to produce drenching rainfall on Thursday.  Most models seem to have good agreement on timing of the system but precip amounts vary widely at this time.  Rain will overspread the state from south to north late Wed night into Thurs morning.  Bulk of the rain should be in SE/CR/RV/NE.  Trend of models seems to be a bit to the west with the precip so we included rain in all zones but kept the wording more scattered across the west.  This definitely needs to be monitored in future shifts as some models do not give rain to western zones.  However, with plenty of moisture, felt it was better to overdo the precip at this time.  Also put breezy in the wording across the eastern third of the state on Thurs due to tight pressure gradient.  Precip amounts could top 2+" across the east on Thurs so flooding concerns should also be monitored.  Rain should move out of area by Thursday night with a few lingering showers possible across northern zones on Friday morning.  Large dome of high pressure builds in by the weekend with upper level trough bringing the 0c line at 850mb into the northern half of the state by Sat/Sun.  This will allow daytime highs to finally feel like fall with temperatures in the 40s across the north, 50s for the majority of the state and 60s in SE/CR.

Forecast Discussion

Synopsis...

Rainmakers are in the picture early in the day Tuesday and on Thursday. Temperatures remain seasonable, with low temperatures later in the forecasting period possibly dropping below 40 degrees. More details to follow.

Short term...

A fairly strong low pressure system will push its way through the commonwealth during the morning and early afternoon on Tuesday. Pretty much every where in the state will see some measurable amount of precipitation during this time. The system though, as indicated by the 18Z GFS run, passes from the west through the north central. This location swill create a steep temperature gradient, generated by SW flow to the east and NE flow in the NNW. Temperatures will remain in the 50s in the west, with highs in the east easily making it into the 70s. High pressure builds behind this system for Wednesday. Although an isolated shower is still possible from this system, I expect most areas to be partly-mostly cloudy during the day.

Long Term...

A 500mb minimum area of voriticity associated with the system moving through Tuesday will generate a low pressure system to the south. This system is expected to merge with a weak tropical low, and move up the coast in time for Thursday. Areas in the eastern part, indicated by the WRF, could receive a very heavy rainfall starting early in the morning Thursday. The proximity of this coastal low pressure system will determine how much of the state sees rain. Once this system vacates, a pocket of dry air will build in behind it. Temperatures will remain a bit warmer on Thursday, but will begin to drop significantly once the low passes. Saturday's highs, according to MOS, are generally kept in the 50s, with lows dropping below 40.


Andrew Dzambo


We Are... Penn State Meteorology

Technical Discussion (Monday Afternoon Shift)

Synopsis…

A large upper level cut off low is going to remain entrenched over the southern portion of Appalachia for the majority of the work week. This system will create a series of surface lows that will move up the east coast creating mild temperatures and persistent rainfall.

Short-term (Monday through Wednesday)

Although we are currently in a brief break from the wet weather, I expect the rain to reform overnight at that cut off low continues to spin over the eastern seaboard. The rainfall will be widespread and heavy, especially overnight tonight with the non-mountainous regions (SE, CR, NS) seeing the heaviest rainfall. There is all chance of thunderstorms associated with this precipitation shield as denoted by moderate Lifted Index values over south eastern portions of the state. Although, severe weather is not expected with this system, if a storm cell gets lucky and heads up the Chesapeake or Delaware Bay, we could see the possibility of a severe thunderstorm in the South east tonight or tomorrow. The upper level low will dissipate Tuesday night allowing for clearing overnight and the chance for a bit of sunshine on Wednesday. Depending on the amount of sunshine on Wednesday, MOS could end up being WAY TOO COLD FOR THE WEDNESDAY HIGH TEMP.

Long-term(Wednesday night through Friday)

The cut off low will strengthen overnight on Wednesday as it pulls in energy from a trough making its way through Canada. However, the GFS and WRF have this occurring at drastically different parts of the country. The WRF has the low developing over Alabama and Mississippi and this allows for the system to pull in more moisture from the Gulf causing heavy rain for Thursday and into Friday. The WRF also pulls in the moisture from what may be TS/Hurricane Nicole, only adding to the Heavy rainfall. The GFS has the low developing over Georgia and South Carolina which causes the majority of the moisture to stay off to the east of Pennsylvania with only the SE region seeing significant rainfall on Thursday and Friday. Since the WRF has been trending East and the GFS is now trending West, we decided to split the difference and forecast somewhere between the two model runs for now and allow for the shifts after us to pin down the forecast as our uncertainty reduces.

Longer-term:

This system will finally clear out on Saturday, but look at the 540 height line as well as the 0 degree Celsius line below Pennsylvania this weekend. Although, I doubt the lake effect snow the models currently are showing will actually occur, it will be quite chilly by the end of the weekend

RJ Patrizio

9-27-2010 Technical Forecast

Technical Discussion

Synopsis…

A potentially significant long-term rain event is in store for this week.

Short-term (Mon. – Wed. afternoon):

Showers are moving into Pennsylvania Sunday night, and a more steady rain is expected for the afternoon.  The models have the heaviest of this first batch of rain west of Philadelphia and Scranton but otherwise across the entire state.  The second batch of rain will have a more easterly component to its direction and affect the entire state at some point.  There is a slight chance that a few showers could linger on the backside of the system on Wednesday for Erie and Bradford, maybe as far south as Pittsburgh.  Total precip was determined using SREF plumes.

Long-term (Wed. night. – Fri.):

How strong will the second cut-off low be?  The 12Z (Sunday) and 0Z WRF indicate a strong cut-off that forms another surface low Wednesday that spreads more heavy-at-times rain across Pennsylvania Wed. night and Thursday.  The CMC shows a somewhat strong cut-off that does not form the Wed. surface low but directs the tropical system more over land.  (I assume the WRF would also show that if it went out longer.)  But the GFS has barely any cut-off low at all and pulls the tropical system off-shore.  Hence the forecasts for Thursday and Friday are as unspecific as "A chance of rain."

Longer-term:

Check out the 540 line diving through Pennsylvania during Saturday night on the GFS, with a MOS low of 39 at KUNV.  Brrrr... 

Scott Sieron

Technical Discussion

Synopsis….

An approaching low pressure system from the southwest will bring moderate to heavy rainfall to the state Monday into Tuesday.  An associated sharp cold front will move through the state on Tuesday, creating conditions favorable for an isolated thunderstorm or two along with the rain showers in parts of the state.   High pressure will build into the region on Wednesday, allowing for drier and seasonable conditions to set in for the remainder of the week.

Short Term….

Generally, the models in the SREF ensemble (9 UTC run) were in decent consensus on the arrival of the precipitation to the southern portions of the state by Monday morning, although a few models were a little bit behind.  The models seem to be projecting the most significant precipitation heading into the state Monday into Tuesday.  This is also shown in the SREF plumes, which, as an example, for State College have anywhere from .2 to .4 inches 3 hour precipitation totals projected Monday into later Tuesday, with the average around up to .2 inches.  The 1200 UTC run of the WRF has most of the precipitation clearing out by very early Wednesday morning, which is supported by the SREF plumes as well.  We were not hesitant in predicting moderate to even heavy rainfall for the majority of the state during this time period due to the aforementioned evidence.  We were very conservative, though, in mentioning the chance of an isolated thunderstorm or two on Tuesday in association with the passage of the cold front, due to the less than impressive MUCAPE plumes value for much of the state, except perhaps the south-eastern portions.

Long Term….

There was not much uncertainly in the long-term forecast, due to the high pressure settling into the region after the low pressure system and cold front move off to our north-east.   Few issues were had with temperature projections after the passage of the cold front—we projected around seasonable temperatures.  It was evident on numerous models (WRF, GFS, etc.) that there will be dry air over the region as well at least later Wednesday into Thursday in association with this high pressure.