textproduct: Gray - Portland

WHAT HAS CHANGED

Snowfall amounts have increased for Sunday into Sunday night as an area of low pressure tracks southeast of the Gulf of Maine. A Winter Weather Advisory has been issued for southern New Hampshire and coastal Maine.

DISCUSSION

KEY MESSAGE 1 Discussion...

Multiple pieces of vorticity rotating through a long wave trough sliding east Sunday continues to lead to tricky snowfall forecast through Sunday night. Confidence is high that an area of low pressure will develop offshore of the Carolinas Sunday and track northeastward outside the 40N/70W benchmark Sunday evening. Models seem to be converging on the northward extent of the precipitation shield pressing as far north as the mountains. Prior to the arrival of this precipitation shield, modest forcing of ascent acting upon a stalled boundary will bring increasing chances for snow by mid morning into early afternoon with accumulations generally less than 1 inch during this period. There may be a lull in precipitation early afternoon with the northern extent of snow from the offshore system pressing into the area from mid afternoon through Sunday evening.

Latest NWP guidance keeps the heaviest snow just offshore where there is strong lift and decent moisture through the dendritic growth zone. Thus, there is some concern that a slight northwest shift in the track will bring higher than forecast snowfall amounts near the coast. Nevertheless light snow is expected across the interior from late Sunday afternoon through midnight Sunday night and periods of moderate snow across southeast NH and far SW Maine. I have issued a Winter Weather Advisory for southern NH and coastal Maine where snow will likely persist the longest with amounts around 4-6 inches. Snowfall amounts will diminish to the north and west of the Advisory with little to no snow accumulation north of the mountains. Winds will generally be light during the snowfall. Temperatures may creep up around the freezing mark near the coast and southern NH during the afternoon and then will fall into the 20s Sunday evening that may allow for refreezing of wet surfaces. Snow will taper off NW to SE Monday morning.

The final piece of vorticity will approach from the west Monday. This set up with the offshore low is favorable for an inverted trough to develop that may result in snowfall continuing through Monday morning while confidence in where/if this inverted trough develops is low.

KEY MESSAGE 2 Discussion...

Long wave trofing will move across the region on Tuesday with H8 temperatures around -20C. This will result in below avg highs with temperatures into the teens and 20s from north to south. Low temperatures will be into the single digits either side of zero. Wednesday will feature similar temperatures with a chance for some light snow late day or at night as a weak wave of energy arrives from the west. Flow will briefly switch to the SSW on Thursday, allowing for a warmer day with highs into the 30s south of the mtns. This will be short lived though as colder air looks to return Thu night through the upcoming weekend.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

Through 00z Sunday...Cigs will lift and scatter through the rest of tonight. Mainly VFR then prevails through Sunday morning.

Cigs then thicken and lower late Sunday morning, with restrictions likely working into coastal/southern NH terminals in the afternoon and snow develops. Snow and low cigs through Sunday evening likely bring at least IFR.

Outlook...

Sunday Night: IFR for most terminals in snow. More northern terminals like LEB and HIE may remain MVFR is precipitation stays south.

Monday: Precipitation comes to an end and cgs lift to VFR. HIE may remain MVFR in upslope clouds and -SHSN.

Tuesday: VFR conditions for most terminals but upslope flow may result in some MVFR CIGS at KHIE. W wind gusts up to 25 kts possible.

Wednesday: Chance for some -SN and lower ceilings but forecast confidence is low.

Thursday: VFR conditions likely for most terminals.

MARINE

Winds will continue to subside through day break while seas remain elevated around 5 feet offshore of the Mid Coast through Sunday. Low pressure passes southeast of Cape Cod late Sunday bringing a wind shift out of the northeast and then north Monday morning. These winds may approach 25 kts briefly Sunday night. The next low pressure system passes north of the waters Monday with increasing SW flow with SCAs likely needed by Monday afternoon.

SCA to near gale force gusts will likely linger Tue through the remainder of the week along with elevated seas. Freezing spray will also be possible as Arctic air moves over the waters.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Winter Weather Advisory from 10 AM this morning to 1 PM EST Monday for MEZ018>028. NH...Winter Weather Advisory from 10 AM this morning to 1 PM EST Monday for NHZ010>015. MARINE...Small Craft Advisory until 7 AM EST Monday for ANZ150-152. Small Craft Advisory until 8 AM EST this morning for ANZ154.