textproduct: Gray - Portland

WHAT HAS CHANGED

Based on observed trends have cancelled the Small Craft Advisories within the Bays while extending the Small Craft Advisory over the outer waters until 11 AM.

KEY MESSAGES

1. Snow likely tonight into Saturday with light to moderate accumulations.

2. Gusty westerly winds are likely Saturday as low pressure departs the region.

3. Ice jams remain in place across some area rivers but the cooler temperatures should largely allow them to remain in place until early next week. A stronger low pressure system is expected to impact the region late Monday into Tuesday morning, bringing potentially locally heavy rainfall and strong winds.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Low pressure is forecast to track thru the Great Lakes and send a warm front lifting into northern New England Fri evening. Forecasts have trended towards more of an occlusion of the primary low pressure and a secondary low trying to take shape in the vicinity of Penobscot Bay. Model guidance even has mid level centers attempting to close off over eastern portions of the forecast area. The end result is longer and stronger forcing for ascent across at least parts of the local area. Now exactly where that mid level deepening occurs will have big impacts for the southward extent of moderate snowfall potential. Currently that gradient is bouncing around central NH into the Casco Bay region. North of that line several inches of snow is possible, along and south of that line is much lower confidence. It could be around an inch and then a dry slot or it could be 2 to 4 inches. Given that we have had a pretty significant melt out and winter is returning it is probably a good idea to remind folks that it is still climatologically winter and I have opted to issue a Winter Weather Advisory for the higher confidence snowfall areas.

KEY MESSAGE 2 DESCRIPTION...

As low pressure departs the mixing heights will increase into Sat afternoon. Forecast soundings suggest that most locations in the forecast area may mix to around 30 to 40 kt. Surface gusts around 30 to 35 mph seems fair if that ends up the case. Winds would increase first across southern parts of the forecast area in the early afternoon, and gradually expand northeastward thru the evening.

KEY MESSAGE 3 DESCRIPTION...

A deep H5 trof axis is progged to move east from the Great Lakes region into New England on Monday into Tuesday while becoming negatively tilted. During this time period, a sfc low will move near the St. Lawrence River Valley, sending a sfc warm front northward as deep south-southeasterly flow brings increasing tropical moisture northward. A trailing cold front will then cross early Tuesday, bringing a wind shift to the west along with the return of drier air.

Increasing WAA as the low tracks west of the area will make this primarily a rain event but a brief period of mixed wintry precipitation is possible early Monday morning, mainly from the foothills and points northward. Anomously strong IVT and precipitable water is likely with the ECMWF EFI and especially the NAEFS showing a strong signal for a high moisture content environment with sfc dew points potentially climbing into the 50s. These high dew points combined with strong S-SE winds and moderate to locally heavy rainfall will likely lead to rapid snow melt where snowpack remains with temperatures climbing well above freezing even in the mountains. The runoff from the melting snowfall combined with the rainfall and remaining river ice will introduce some flood potential, mainly due to possible ice jams. That being said, river flows remain low given ongoing drought conditions and therefore storage space is higher than usual.

NBM and ECMWF ensemble means show a rather high probability (>=70%) for at least 1.00" of rainfall but probabilities for >=2.00" are much lower (~20%). This is mainly due to the short residence time of this system.

Another thing that will need to be watched is the potential for strong S-SE winds. Forecast soundings and low-level wind fields show the potential for 80-90 kts at h925, which given mixing could allow for sfc gusts between 40-50 mph. Even though trees remain bare, winds out of the SE tend to cause more in the way of issues in our area and therefore power outages are possible.

Following this system a return to more seasonable temperatures and mainly dry conditions look likely for the remainder of the week.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

Through tonight... Widespread VFR conditions thru morning, but along and north of the mtns local IFR or lower possible in upslope snow showers. VFR conditions continue into this evening when a warm front will spread light snow across the forecast area. Areas of IFR or lower will develop and continue into Sat morning.

Outlook:

Saturday: Conditions return to VFR Sat afternoon but westerly winds will become gusty. Surface gusts up to 30 kt possible at all terminals.

Saturday Night: VFR conditions expected. Westerly wind gusts up to 25 kts possible early before becoming 10-15 kts late.

Sunday: VFR conditions expected. Light and variable winds.

Sunday Night: Gradually lowering ceilings with -RA possible towards dawn on Monday. Increasing SE winds late at 10-20 kts.

Monday/Monday Night: IFR-LIFR ceilings in RA. SE winds 25-35 kts possible.

Tuesday: Improving conditions back to VFR with winds becoming W at 15-25 kts.

MARINE

Gusty northwesterly winds are expected behind a cold front thru the morning. SCAs remain in effect for all waters. Winds and seas diminish thru the afternoon. A warm front will lift across the water this evening and overnight with southerly winds increasing in response. SCAs are possible for all waters. Winds will turn sharply westerly tonight into Sat. These wind gusts may approach gale force outside of the bays.

At least Gale force S-SE winds are likely Monday into Monday night with potential for storm force winds as low pressure tracks west of the waters. Seas of 15-20 ft are possible outside of the bays. Winds and seas will then gradually begin to reduce on Tuesday as winds become westerly.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Winter Weather Advisory from 11 PM this evening to 2 PM EDT Saturday for MEZ007>009-012>014-033. Winter Weather Advisory from 11 PM this evening to 11 AM EDT Saturday for MEZ020>022-026>028. NH...Winter Weather Advisory from 11 PM this evening to 2 PM EDT Saturday for NHZ001>004. Winter Weather Advisory from 11 PM this evening to 11 AM EDT Saturday for NHZ005-006. MARINE...Small Craft Advisory until 11 AM EDT this morning for ANZ150- 152-154.