Marine Weather and Tide Forecast for Lancaster, PA

KEY MESSAGES
1) Light snow spreads quickly NE and across southeast PA last tonight and Sunday morning as low pressure develops off the east coast.

2) An arctic blast Monday will produce little snowfall but lead to dangerous, sub-zero wind chills Monday night and Tuesday.

3) Colder than normal temperatures continue through the end of the month with storm chances (light to moderate snowfall) every few days.

DISCUSSION
KEY MESSAGE 1: Light snow spreads quickly NE and across southeast PA last tonight and Sunday morning as low pressure develops off the east coast.

Fast moving nose (thermally indirect branch) of a strong upper level jet (170 kts) will bring a several hour shot of Meso-B scale UVVEL and light to briefly MDT snow across the Lower Susq Valley late tonight through the mid morning hours Sunday.

A sharp gradient/cutoff in snow accumulations will be seen on its nrn and western edge respectively across the Middle Susq Valley and Scent Mtns as several weak waves of low pressure develop just off the east coast along a stalled out frontal boundary.

Very light snow will initially develop in the typical upslope regions of Cambria and Somerset Counties after midnight as the upper trough arrives before energy transfers to a developing area of low pressure off of the east coast. This system will bring light snow to the Lower Susquehanna Valley tonight and tomorrow, with portions of York and Lancaster County likely seeing up to 3 inches of snow. We will likely be issuing a Winter Weather Advisory for portions of southeast PA to highlight this potential.

KEY MESSAGE 2: An arctic blast Monday will produce little snowfall but lead to dangerous, sub-zero wind chills Monday night and Tuesday.

A cold front tracks through Central PA on Monday. Though moisture will be limited with the front, there may be some light snow that accompanies it. In its wake, surface and steering winds out of the southwest or west- southwest should keep lake effect snow bands directed into New York with mainly dry conditions in Pennsylvania. Min wind chills Monday night/Tuesday morning range from 0F in the LSV to minus 20F over portions of the Alleghenies. The current forecast would support a Cold Weather Advisory in the Laurels and northern mountains (approximately along, west, and north of the I-99/I-80 corridor)
and possibly an Extreme Cold Warning in the Laurel Highlands.

Forecast highs in the + single digits to ~20F on Tuesday are 15 to 25 degrees below the historical average, though not quite record setting. Wind chills likely stay below zero west of I-99 and north of I-80 through Wednesday morning. Low- level flow becomes more westerly by Wednesday, which will help to direct lake effect snow bands back into northwest PA.

KEY MESSAGE 3: Colder than normal temperatures continue through the end of the month with storm chances (light to moderate snowfall) every few days.

A persistent west coast ridge and east coast trough pattern in the upper levels will ensure the continuation of below normal temperatures through the end of January. Periodic waves of low pressure traversing the High Plains and Great Lakes mean we'll see frequent opportunities for precipitation. Most of the storm systems appear weak and relatively disorganized at this point, which would favor the highest snowfall totals occurring in the typical upslope and lake effect snow regions of the Laurels and northwest mountains through the end of January.

AVIATION /01Z SUNDAY THROUGH THURSDAY/
MVFR CIGS will continue/develop from the SW to NE across the NW Mtns and Laurel Highlands this evening and expand to the east around or shortly after midnight.

LIFR to VLIFR in Patchy Dense FOG will linger at/around KLNS for the next 2-3 hours before cloud cover overspreads the SE PA airfields and a light breeze helps to improve the low vsbys and ceilings.

The evening lull in light snowfall will transition to some light snow around midnight in Somerset County and by 3 or 4 AM in Adams/York/Lancaster as a fast moving upper level disturbance approaches from the SW accompanied by thickening layered clouds and lowering cloud bases. The light snow will likely fall at moderate intensity (~1/2SM) at times in the 09-13Z Sunday window, especially in York and Lancaster Ctys.

The heavier snow and LIFR conditions will be mainly S of MDT and MUI. These area will likely (90%) see LIFR for 3-6hrs. The snow and low cigs/visby will lift to MVFR or better in the early aftn. The winds will become less gusty over the W early tonight. The light wind in the east will prevail into Sunday morning.

Outlook...

Mon...Cold fropa. West winds gusting over 30kts. Restrictions likely with snow showers especially west.

Tue-Thu...Light snow/IFR possible in the NW at times.

CTP WATCHES/WARNINGS/ADVISORIES
Winter Weather Advisory from 3 AM to 3 PM EST Sunday for PAZ065-066.