11-15 Technical Forecast Discussion

Nov. 15. 2011 Technical Forecast Discussion

Synopsis:

A slow moving front will bring on and off showers for the rest of the day on Tuesday with a steadier rain arriving later Tuesday night. The rain will be moderate in intensity for the morning hours on Wednesday and tapering off to light rain showers by Wednesday afternoon. The clouds will linger into Thursday with some showers possible for southeastern portion of the state. The clouds will gradually clear up on Friday with mostly sunny skies to start the weekend.

Short-term (Tuesday evening-Wednesday evening)

A cold front accompanied with a strong jet stream and vigorous upper-level shear that brought lines of thunderstorm squalls through the Ohio Valley and Pennsylvania will stall just south of the region Tuesday evening. Moisture riding along this stationary front will create periods of rain showers to the region. As the broad 500-mb upper-level trough situated over the upper Midwest continues to swing packets of vorticity maximums south-eastward, a series of low pressures will form over the Tennessee Valley and slide north-eastward.  As result, overrunning events will occur and bring some rain later Tuesday night and into Wednesday morning. This overrunning event is going to be a rain event because the temperature is just simply too warm for any frozen precipitation. The rain will continue throughout Wednesday and taper off to showers later in the afternoon. We can't rule out the possibility of seeing more rains falling Wednesday night, as GFS suggests.

However, this synoptic setup is a close call for a northeast snowstorm: a strong upper-level trough advecting positive vorticity southward, a vigorous cut-off upper low swinging through the southern states, a weak ridge attempting to form over the Rockies to amplify the upper-level trough. The only ingredients that are missing are the phasing of the northern and southern upper-level trough and the amplification by the west coast ridge. These can all be attributed to the unfavorable locale of the polar vortex situating over Alaska, bring waves after waves of troughs to the west coast and keeps the heights over the southeastern U.S abnormally high; allowing for a rather zonal flow across the country. In the case with the recent October snowstorm, the trough dipping down from the upper Midwest was just marginally deep enough to phase with a southern branch of the jet stream to cause a robust coastal storm. If anyone reading this has attended Dr. Uccellini's, the Director of NCEP, presentation featured by PSUBAMS, he or she should pay close attention to rapidly changing forecasts this winter. As Dr. Uccellini theorized, the phasing of the troughs tends to be harder to predict during La Nina ENSO conditions. Last winter we had a moderate La Nina and the post-Christmas snowstorm occurred. The ENSO is again forecast to be a moderate La Nina and we already have seen its possible effects on cyclongenesis prediction in the recent October snowstorm, where models failed to converge.

Mid-term (Thursday-Friday):

That was just some side notes, now back to forecasting. The rain showers should have moved out of the region by Friday morning except for extreme southeastern Pennsylvania might still see some lingering showers. The clouds will linger for most of the state and the temperatures will be cooler comparing to what the region will see on Wednesday. The winds might also be quiet strong behind the cold front, so please bundle up. The clouds will gradually clear out Thursday night if the forecast goes as planned. We should mostly sunny skies on Friday as a ridge of high pressure builds in over the region.

Long-term (Saturday-Sunday):

                Calm weather conditions will persist into the upcoming weekend as the overall weather pattern favors a dominant southeast ridge. The temperature will warm up to above normal as warm air is being advected from the south by the large area of high pressure. The next chance for some precipitation will be later on Sunday as another cold front approaches from the west.

Written by David Wang