Tuesday Night Technical Discussion

Synopsis:

Low pressure will make it's was off the coast leaving a westerly flow for the region in it's wake for Tuesday night through Thursday, bringing cold temperatures. High pressure will move over the Mid Atlantic Region Friday and bring a southerly component to the wind and allow for some slight warm advection. Another system will affect the region Saturday morning through the rest of the weekend bringing a chance for mixed precipitation across the state.

Short Term: (Tuesday night through Thursday night)

A mean westerly flow Tuesday night through Thursday will produce frigid temperatures. The highest temperatures will only reach the mid to upper 20's for the entire state both Wednesday and Thursday and low temperatures could reach single digits Thursday night for higher elevations in western portions of the state indicated by the 18Z 12km NAM. The potential for lake effect snow Tuesday night and Thursday night will be limited to the extreme northern and northwestern portions of the state or even north of the state due to a mean westerly wind with a southerly component at times. Some lake effect snow showers are possible for northwestern counties Wednesday night due to a slight northerly component to the mean westerly flow.

Long Term: (Friday into the weekend)

High pressure passing to the south of the state will introduce a south westerly wind to the region and help advect some warmer temperatures into the state. The southern half of the state could reach high temperatures in the low 30's on Friday as indicated by the 18Z 12km NAM. A system will approach the region from the southwest and begin to impact the state Saturday morning with light snow showers but a changeover to mixed precipitation is possible as warm flow off the ocean is pushed inland. The system looks to have two centers of low pressure, thus a Miller Type B storm, and the coastal low will be most dominant indicated by the 18Z GFS. This will keep most of the heavy precipitation to the south and east of PA. The system will move off the coast Sunday and leave behind cold advection, and a north to northwesterly flow providing a chance for lake effect snow showers Sunday night for northwestern counties.

~ A.J. Herbert

Technical Discussion for Wednesday Night Shift (12/4)

Synopsis:

A tricky winter weather pattern will be set up across Pennsylvania for the next several days. A warm front will bring unseasonably mild temperatures to the state Thursday, while a cold front entering Thursday afternoon will start a two-day period of precipitation, as the front will hang across the state. Saturday is a dry, partly sunny day, while Sunday and Monday we could see another shot at a mixed bag of precipitation across much of the Commonwealth.

Short Term: through Friday morning

As of 0300z, the WPC surface analysis placed the warm front associated with a deepening low in the upper Midwest was just starting to cross into southwestern PA. It will continue to move northeast throughout the night, thus overnight low temperatures are expected as the front crosses. After the frontal passage, mild air will push in quickly (Wheeling, WV is at 57 degrees directly behind the front).

This means that temperatures will warm up significantly on Thursday, despite lingering clouds once again. Most locations will make it at least to the upper 50s, with some locations in southern PA pressing towards the mid-60s for a high, more than 20 degrees above normal. The cold front should sweep into western PA by mid-afternoon, as indicated by a general model consensus. Light precipitation falls ahead of the front, but the bulk will follow behind the front, according to model guidance. As the front gets hung up across the state, rain will persist overnight Thursday night into Friday morning.

Long Term: Friday – Monday

A shot of arctic air will come in during the day on Friday, cooling temperatures aloft first. While some locations could see a brief amount of mixed precipitation, most soundings for late Friday afternoon show a quick changeover to all snow. Most of the snow showers we see across the state Friday night will be the result of shortwave disturbances along the deep trough set up across the US, and their strength and track are not exactly certain yet, thus accumulation amounts vary widely across the different models, including among SREF members. It does seem likely that much of the state will wake up with at least a coating of new snow Saturday morning.

The stalled front will finally move away early Saturday, leading to much of the state seeing the first significant sunlight all week. The cold air will remain in place, however, so temperatures will stay at or below seasonal normals and will dip quite low overnight. For Sunday models indicate warmer air beginning to overrun into Pennsylvania by the afternoon hours. As the atmosphere moistens, there becomes great concern for significant ice accretion in parts of the state Sunday night into Monday. Soundings from the 0z GFS show the greatest likelihood for sleet changing to freezing rain around midnight on Monday. As temperatures continue to warm, the precipitation changes over to all rain Monday morning before ending Monday afternoon. Although the exact temperature profile (and therefore precipitation type) is not nailed down yet, the forecast beyond Monday's storm becomes a bit less active and much colder.

-Devin Boyer

Synopsis: 

A warm front will lift north into Pennsylvania today, bringing along above average temperatures throughout the state.  A cold front from the west will move through Pennsylvania late Thursday afternoon into late Thursday night, before it becomes nearly stationary in the Mid-Atlantic region Friday.  Waves will accompany the stationary front, which is set to bring mix precipitation into the central regions of the state, and light snow accumulations for the western regions of the Commonwealth Friday afternoon into Friday evening.

Short Term:  (Wednesday night through Thursday night)

The fog and the low clouds in the central region have scoured out in the warm sector.  The warm sector has made its way into the area, bringing temperatures 5-10 degrees above normal.  Drizzle and fog will appear tonight due to warm air advection from the south ahead of the cold front.  High temperatures tomorrow could range from 10-20 degrees above normal.  The cold front will arrive into Western Pennsylvania around 18z tomorrow.  Scattered rain showers will arrive ahead of the cold front tomorrow afternoon.

Long Term:  (Friday into the weekend)

An arctic cold front will push eastward toward Pennsylvania Friday night, and bringing artic cold air for Saturday.  This would be the first out of the two mixed precipitation events.  A mild southwest air flow will support rain for most of area Friday, with a changeover to frozen precipitation   from Northwest Pennsylvania to Southeastern Pennsylvania Friday night.  As the frontal boundary moves eastward, colder air will overtake the warm air.  As shortwaves ride northward along the front Friday, air columns will be cold enough to support all snow, which could create accumulations of 3 – 5 inches Friday night.  Another mixed wintry precipitation event will occur on Sunday afternoon.  Medium range models and ensemble guidances depicts a low pressure developoing in the Tennessee valley, and lifting northward during late Saturday into Sunday.  On Sunday, deep moisture and warm air advection aloft over a sub-freezing surface temperature will support frozen precipitation for the area.  GFS and ECMWF guidance shows that freezing rain accumulations could occur early Sunday afternoon into Sunday evening for the central regions of Pennsylvania.  As the models update overtime, the confidence level of the storms track & precipitation type will increase.

~ Chad

Tuesday Night Technical Discussion

Synopsis:

High pressure will move off the coast as a low-pressure system to our west will move north of the region. The state will be in the warm sector of the system and temperatures will be warm for the middle of the week. The low-pressure's associated cold front will stall across the state Thursday and Friday, as well as into the weekend, bringing widespread precipitation. A second system will swiftly make its way from the gulf and provide a chance for mixed precipitation for the end of the weekend and for the beginning of next week.

Short Term: (Tuesday night through Wednesday night)

Tuesday night there will be a slight chance for some light freezing rain or drizzle for extreme northern PA, otherwise cloudy conditions will persist overnight and low temperatures will remain in the 30's for the majority of the state as indicated by the 18Z 12km NAM. As the low pressure and it's associated cold front to our west slowly approaches the region, temperatures will begin to warm for Wednesday, as we will be located in the warm sector with a southeasterly flow. This southeasterly flow could provide enough moisture to produce a few scattered rain showers for higher elevations located in the eastern counties as the moist air ascends the mountains.

Long Term: (Thursday into the weekend)

Temperatures will continue to warm Thursday with high temperatures ranging from the low to mid 50's for northern counties to low 60's for southern PA indicated by the 18Z 12km NAM. The 18Z NAM, as well as the 18Z HI-RES 4km NAM and the 21Z SREF Mean, have the frontal precipitation reaching southeastern portions of the state by 18Z Thursday, but the 18Z GFS is a bit slower with the precipitation not reaching southern counties until 03Z Friday. Pieces of energy will make their way from the southwest and move northeasterly along the stalled cold front across the state beginning Thursday night. This will create bands of precipitation to be oriented from the southwest to the northeast from Thursday night into the weekend, as pieces of energy continue to move northeast along the cold front and bring widespread rain showers across the state. As the stalled cold front finally makes its way off the coast, cold air will move into the state and provide a chance for some snow showers in northwestern counties Friday night as the precipitation exits the state to the southeast. A quick moving system from the gulf will move just west of the state and bring a chance for some mixed precipitation Sunday night through Monday. 

 ~ A.J. Herbert