textproduct: Gray - Portland

WHAT HAS CHANGED

Quick update to expire the Small Craft Advisory. Forecast remains on track.

KEY MESSAGES

1. A potent low pressure system will bring some locally heavy rain and gusty winds to most of the forecast area early Monday morning through Tuesday morning. Some localized flooding is possible as ice jams loosen and the remaining snow melts.

2. Slippery conditions possible late tonight and Monday morning in the winter weather advisory area as light freezing rain is expected, mainly at higher elevations.

3. Gusty winds remain behind the departing system Tuesday, with otherwise little to no impactful weather through mid-week. Some guidance suggests the next chance for widespread precipitation around the Saturday timeframe, but there remains a wide envelop of outcomes.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... As 00Z data continues to be received models remain in reasonably good agreement on most aspects of Monday's storm. Rain looks to just be beginning in southwestern New Hampshire around or after midnight and overspreading the area through the rest of the night and Monday morning. At this point ensembles suggest PWATS are around 0.75 to 1" which suggests some locally heavy rain is possible, particularly in southern New Hampshire where the higher values are present and also eastern slopes of the White Mountains. The CAMs also agree there could be some instability here as well leading to convectively enhanced precipitation and maybe a period of thunder.

Activity looks to become more showery and drizzly Monday afternoon so some locations may see a lull (or a few lulls). However, a strong low level jet continues to transport deep moisture into the area so showers are going to be able to put down some water with PWATs rising into the 1.25 to 1.40" range. The afternoon is also when winds gusts may ramp up, but this is probably the most uncertain aspect of this storm. There is going to be a very strong low level jet moving overhead (around 90 kts at its peak Monday night), but forecast soundings show a stout inversion forming and persisting during this time as well. Models like the NAM and Euro are on the conservative side keeping gusts generally in the 30-40 mph range which is reasonable with just the tight pressure gradient that will be present alone. Models like the HRRR and the GFS are a little more bullish with 50-60 mph gusts. I feel like the solution is more of a hybrid, 30-40 mph gusts a good ballpark for what will likely be frequently observed with the inversion keeping us from the higher values. However, convective mixing would briefly negate the effect of the inversion meaning those values of 50-60 mph may be a good estimate for what could be observed briefly. I could see wind headlines eventually being needed, but confidence is still too low with the looming inversion potentially keeping the frequent gusts just below advisory level. Best chance at wind headlines would likely be the Midcoast of Maine.

The second slug of rain comes Monday evening as the cold front approaches. PWATS remain elevated and the NAMNest continues to suggest enough instability for continued convective enhancement, so heavy rain remains a factor until the early hours of Tuesday morning when it begins to taper off. When all is said and done 1-2" still looks like a good estimate for total rainfall with much of the area expected to fall right in the middle. Ensemble probabilities are now 90 to near 100% for the majority of the forecast area to receive over 1". Areas north and west of the mountains may fall just short as they are less likely to see the enhanced rainfall. The areas that ensembles continue to target for greater than 2" are the Midcoast and the White Mountains, with probabilities still in the 60-70% range. It should be noted CAMs are targeting the Midcoast as well. Fortunately the Midcoast should make out just fine. The more concerning area for 2" of rain to fall is the mountains and with the upslope component, enhancement there is not uncommon. Runoff from this could bring some of the more sensitive river forecast points into action or near minor flood stage. The ice jam flooding risk has decreased a bit with observations showing most rivers have flushed out their ice, but some is still present especially in he north so this risk is non-zero and rivers are going to have to be monitored closely. Confidence is not there yet for any flood headlines, but as we are able to determine how rivers are going to react to this runoff those may be needed eventually as well.

KEY MESSAGE 2 DESCRIPTION... The antecedent SFC air mass later tonight and early Monday morning should be cold enough to support light freezing rain across western and northern NH as well as the western ME mountains along with portions of the foothills. Have therefore issued a winter weather advisory for potentially slippery conditions in these areas later tonight into Monday morning. But Monday afternoon it will likely warm up enough to allow just plain rain.

KEY MESSAGE 3 DESCRIPTION... As the aforementioned system exits the region Tuesday, gusty northwesterly winds will pick up as a pressure gradient sets up in between the departing system and incoming high pressure from the west. In addition, a mid-level trough will engulf the northeast, allowing for a quick return to winter. Wind chills look to be in the single digits or colder Wednesday morning as a light northwesterly breeze presides over air temperatures ranging from single digits up north to teens along the coast and south.

High pressure continues through the rest of the week, with temperatures gradually warming as the mid-level trough makes a slight realignment to the west. The next chance for precipitation looks to be next weekend.

AVIATION /14Z SUNDAY THROUGH THURSDAY/

Through 12Z Monday...Westerly winds relax into this morning with VFR conditions. VFR should prevail through this evening. Conditions deteriorate overnight tonight as ceilings lower to MVFR ahead of our next weather system. IFR conditions are possible as early as 08Z Monday as precipitation begins to enter the area. Outlook...

Monday-Monday night: High confidence in widespread IFR cigs as a storm arrives with RA and FG. LLWS is likely, with strong SE to S surface gusts along at least coastal terminals.

Tuesday: Cigs should be improving to VFR from MVFR through the morning. W gusts 30-35 kts through the day.

Wednesday-Thursday: VFR expected, no sig wx.

MARINE

Conditions turn fair on the waters today as high pressure crosses the area. Overnight tonight conditions will quickly deteriorate as a strong weather system approaches. Easterly gales are possible by Monday morning, with near storm force winds possible by Monday night. This will also build wave heights to 15-18ft at their peak Monday night.

Gale force westerly winds Tuesday morning continue as low pressure departs from the Gulf of Maine. Abnormally high surf is expected, with seas of 10-16ft. Seas drop to 8-12ft by the end of the day. Conditions start to improve by Wednesday morning, with SCA level westerlies and 6-10ft seas expected by then. SCA conditions continue through the day Wednesday. Sub-SCA conditions return for the end of the week as high pressure moves overhead.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Winter Weather Advisory from 3 AM to noon EDT Monday for MEZ007>009-012-013. NH...Winter Weather Advisory from 3 AM to noon EDT Monday for NHZ001>005-007. MARINE...Storm Watch from Monday evening through Tuesday morning for ANZ150>152-154. Gale Watch from Monday morning through Tuesday morning for ANZ153.