Wednesday, January 20, 2010


Welcome to the Winter Stormcast Zone

11 PM THU 1.21.10 UPDATE

FORECAST COLLABORATORS
In MD- Ryan K., PasadenaMatt, Dakota, Winterman, Mr. Foot
In PA- Brisko, Alto, Meteorologist JSTWX from WJAC Johnstown

LATEST IDEAS  From Forecaster Ryan at Sparrows Point HS in Baltimore County:
As the first batch leaves the region, and a small break in the precip occurs, an upper-level low is located near Tennssee with associated precip. This somewhat juiced low should move E/ENE and pass south of the region based off of short-wave analysis. The precip would likely be in the form of snow at this point due to a colder column of air resulting from evaportative cooling at mid-levels. Total accumulations in the central MD region of snow/sleet/freezing rain at this point look to be 1'', but slightly higher amounts are definitely possible

TEAM SYNOPSIS We believe our overall ideas as outlined Wed 1.20 are on target. Supporting our hypotheses are growing moisture plumes north of the Gulf of Mexico. This tells us the eventual storm will come farther north than most models are projecting. Computer model support from the WRF (we call it the "Worf!) also indicated this event will track farther north than is currently anticipated. The WRF and Canadian were the first models to show that the Dec 19 storm would move more north.

STORM TIMING AND IMPACTS
ONSET  Rain occasionally mixed with sleet Thursday afternoon
CHANGEOVER To sleet, then freezing rain by 9PM
ICE ACCRETION Up to .20" by daybreak Friday:
< than .10" ice; mainly sleet/rain mixed: South and east of I-95
.10 to .20" ice: Just west of Washington, Baltimore and Philadephia
SNOWFALL
Light and mixed at onset. As coastal low pulls away Friday, freezing precip changes back to snow, and may accumulate up to 2 inches from central Maryland north and east as well over DelMarVa by Friday afternoon.
SCHOOLS & COLLEGES
Expect delays and closings for schools on Friday. Those who are required to brave the elements and have school-age children at home should make alternate childcare arrangements.

WHAT IS THE NATIONAL WEATHER SERVICE SAYING?

OUR SUMMARY By Thursday Night, precip across the Mid-Atlantic west of the Chesapeake Bay will have transitioned from rain and sleet mixed to freezing rain. Sleet will quickly lower surface temperatures. Until rain changes to snow, a coating of ice up to 1/4 inch will develop overnight into Friday. We expect evaporative cooling to change precip over to snow by daybreak. As a secondary low churns in the Carolinas, this will enhance the "cold air damming" (CAD) wedge across the Mid-Atlantic east of I-81.

Thanks to the team for lots of input and detailed analysis on Tuesday night. A warm winter welcome to Mr. JSTWX of WJAC-6 in Johnstown, PA! Thanks for collaborating with us.




PREVIOUS ANALYSES


WAVE # 1 Arrives Wednesday across central and western MD-Northern Virginia- WV panhandle. Early indications show that temps at 2 meters and 5,000 feet look favorable for snow. Were this scenario to continue developing, there is enough liquid equivalent and recent arrival of cold air could produce light snow across the region by Wednesday night.


WAVE # 2 Rain on Thursday across the Mid-Atlantic transitions to snow and sleet by nightfall. Wintry precip continues into Friday morning for areas west of the Chesapeake Bay and southern Pennsylvania, including the I-95 metro areas of Washington, Baltimore and Philadelphia.


Passage of wave one may permit brief northerly flow to setup as a surface high presses in from Quebec The situation here is more significant ; there may be considerable moisture content in the second system and the open wave becomes a closed low. We also note position of the surface high as being more favorable to generate a a resupply of cold air as a secondary low may take shape over the Carolinas. Liquid equivalents look to range approximately .75" which with 8:1 ratios could yield approximately 4" of snow mixed with sleet by Friday morning.

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