Ph.D. Student - Earth System Science (UC Irvine), B.Sc. - Atmospheric Sciences (Cornell University)
By: Zachary Labe , 3:35 PM GMT on October 11, 2010
The trailing cold front will pass through north to south on Monday and Tuesday leaving behind a drier and cooler airmass for Wednesday. 1016mb high pressure will dominate the region's weather on Wednesday with seasonal temperatures and low dewpoints under clear blue skies. Low dewpoints will allow for a frost potential in the northern valleys of Pennsylvania for Tuesday and Wednesday nights. Meanwhile a trough will be deepening to our west as a shortwave begins to dig along the trough axis in the Ohio Valley. This setup will lead to cyclogenesis along the Northeast coastline as the first nor'easter of the year is underway.
October nor'easters are quite common to the Northeast and small details of their development often foreshadow critical signs to the winter makeup. By Thursday the 1020mb high pressure will lift to our northeast in a favorable location for cold air damming near Nova Scotia while the 850hPa low tracks south of the Mason-Dixon line slowly organizing. As often noted in many situations, the exact track of the 850hPa low is critical to the position of the heaviest precipitation axis and temperatures aloft. Being that it is only early to mid October, 1000-500mb thicknesses will not be of importance for the most part. Current GFS and ECMWF prognostics are in similar correspondace tracking the 850hPa low across northern Maryland. This would position the heavy rain axis about 50-100mi to the north. Current GFS surface rains agree with this rule showing heavy rain in northern Maryland to about interstate 80. More convective rains would be likely south of the 850hPa low track across southern Maryland and the Delmarva.
By Thursday night, the 1004mb surface low will track across Delaware and begin to deepen off the coast. Current Atlantic SST anomalies have waned from their impressive deviations during the summer courtesy of the tropical systems, which stirred a bit of the cooler water wells to the surface. In any case the low will undergo cyclogenesis Thursday night progresses slowly to the northeast. As the low deepens, 1020mb high pressure over Newfoundland will funnel in cooler air into the Northeast with 850mb temperatures dropping to (+1)-(+3)C as far south as Maryland. Under the northeast flow, a thick stratus layer will promote very cool diurnal high temperatures. GFS 2m temperatures suggests highs only in the 50s for most areas. But considering past history of cold air damming, I would undercut those prognostics by a good 5F. Highs will likely be in the mid to upper 40s for most areas north of Baltimore for Thursday and Friday. The higher elevations above 1800ft may even only stay in the low to mid 40s especially towards the Poconos.
The nor'easter is supported by current teleconnections with a negative NAO and favorable phase 5/6 MJO.
10/11/10 06utc GFS shows a 996mb surface low about 25mi off the coast of central New Jersey by early morning Friday. 10/11/10 0utc ECMWF is in similar correspondance, but deepens the low about 75-100mi off the coast.
As the 988mb surface low becomes cutoff late Friday into Saturday, the heavy rain will shift into New England. But before, much of the day will Friday will feature a heavy rain deformation axis along the east coast. Positioning of this exact band is of course in question at this point. Current 10/11/10 morning GFS 700mb RH prognostics show the deformation axis along eastern Pennsylvania as far south as the Mason-Dixon line. GFS/ECMWF mean QPF is generally 1-4in of rain for all of Pennsylvania/Maryland/Delaware with the higher amounts in eastern Pennsylvania on northward. A tight pressure gradient is also likely especially as the low deepens off the coast by Friday morning with a northeast flow dominating. 10m ECMWF surface winds suggest sustained 35-45mph winds especially for coastal areas, but also elevations above 1200ft. Regionwide gusty winds will be likely at times reach advisory 50mph criteria, again especially for eastern and northern areas.
Being that it is early October, rain will be the dominate precipitation type. But a few GFS/ECMWF runs have hinted at cooler air being pulled into the system as the low becomes vertically stacked. 1000-500mb thicknesses drop to 545dm for parts of New England, which may be cold enough for some accumulating snow for elevations above 3000ft especially into the Presidentials in New Hampshire. This threat of elevation accumulating snow will need to be monitored later this week.
Other global models also support the nor'easter threat including the NOGAPS and GGEM, but they are a bit slow for intensification and a bit farther off the coast. None the less I think a general forecast can be made for Thursday into Friday for the northern Middle Atlantic with heavy rain, gusty winds, and cool temperatures. Highs will struggle to reach 50F northwest of the I-95 corridor during this period and gusty winds with rain will make it feel even cooler. Rainfall amounts will likely surpass one inch for most areas in the region with higher amounts likely north of the Mason-Dixon line and east of the State College-Lock Haven line. Coastal flooding is also possible, but should remain minor as winds will have a more northerly component as opposed to southerly. As the low pulls northeast near the 40/70 benchmark, high pressure will move in for a nice Fall weekend with highs in the 60s and lows in the 30s/40s for most areas under blue sky.
"Here north of Harrisburg 2010 statistics"
(Severe Weather Stats...)
Severe Thunderstorm Watches- 13
Severe Thunderstorm Warnings- 10
Tornado Watches- 2
Tornado Warnings- 1
Total Thunderstorms- 20
Flood Watches- 4
Flood Warnings- 4
Monthly Precipitation- 1.70inches (October)
Yearly Precipitation- 33.73inches
Heat Advisories- 5
Excessive Heat Watches- 1
Excessive Heat Warnings- 1
90degree days- 38
Highest Temperature 101F (x2)
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