textproduct: Gray - Portland

WHAT HAS CHANGED

Scaled back on PoPs for early this morning except for the drizzle. Also expaned thunder coverage for later this afternoon and evening as EML advects overhead and aids in a good amount of elevated instability.

KEY MESSAGES

1. A widespread cold and beneficial rain is expected across western ME and New Hampshire today and into tonight. A period of wintry precipitation is also possible across portions of the western Maine mountains.

2. Damp conditions possibly linger into Wednesday, with drier conditions for Thursday.

3. Low pressure passes through on Friday, with cold and windy conditions behind the system for the weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Northeast winds continue to bring colder air into the region early this morning behind a backdoor cold front and high pressure building in to the north and east.

Latest model guidance holds these northeast winds for much of the day, setting up CAD and locking in the colder air. Temperatures will likely be held in the 40s south of the mountains with the exception of southern NH and maybe the ME coast reaching the lower 50s late today. With the NBM having a bias of warming temps too quickly, have continued the trend of the last few forecasts in leaning much more closely on colder guidance. This also keeps temps in the 30s through the day in the mountains and more on that below.

A wave of low pressure approaching from the west will result in precipitation overspreading the area this morning and afternoon, which is likely to continue into tonight. This will be just a cold but beneficial rain (and maybe a rumble of thunder or two with some elevated instability) for the vast majority of NH and western ME, but the even colder air to the north could bring a wintry mix today across portions of the western Maine mountains. The result may be the first coating of snow of the season with guidance favoring northern Franklin over into central Somerset counties, although temperatures are marginal outside of the higher terrain in how much will stick. Forecast soundings also continue to indicate the possibility of sleet or freezing rain, but I think this would be more likely to stay in the higher terrain. Therefore, a winter weather advisory has not been issued.

Forecast rainfall through tonight remains in the 1-1.5" for most of western ME and central and northern NH. Amounts taper off to 0.25" to 0.50" toward southern NH.

Lastly, the HRRR does show a slightly veering of the winds and a downsloping signal north and west of the mountains this morning and afternoon, potentially resulting in gusts of 30-35 mph.

Isolated thunder remains in the forecast for late this afternoon and evening, especially across southern zones as instability increase in the wake of the main stratiform precipitation shield.

KEY MESSAGE 2 DESCRIPTION...

The trough exits by Wednesday morning, with subsidence and height rises ensuing. However a rather pronounced warm nose remains at 700 mb. With low clouds possibly staying in the picture (especially in the northern highlands), unlikely we will be able to fully mix and erode out this stable layer aloft. In addition, soundings show plenty of moisture (likely enhanced from tomorrow's precipitation) that gets ducted/trapped below the inversion. Areas in the higher terrain will likely see some pockets of drizzle/fog, especially during the morning hours. How much clearing occurs further south is uncertain, but generally am cautious to accept the more bullish NBM/global model guidance at this point given concerns over cloud cover/poor mixing depth. Likely will be a raw day for interior zones with temps staying in the 40s again, and 50s-60s closer to the coast/further south. Far southern NH has the better shot at highs near 70 and clearing skies.

Moisture looks to finally mix out Thursday. Conditions look seasonable, with highs ranging from the upper 40s across the north, to low 60s along the coast.

KEY MESSAGE 3 DESCRIPTION...

A strengthening low pressure system will impact the Northeast U.S. on Friday, and keep a lingering influence on the local weather into the weekend. The EPS ensemble members track the low right overhead, while the GEFS members show a further north track, over the St. Lawrence River valley. Both sets of guidance show the low may deepen to around 990 mb, which is on the stronger side for this early in the cold season. The specific track will dictate if any of the region can get into the more mild warm sector, but in any case, another round of beneficial rain looks likely, with generally a half inch to an inch of QPF.

Very strong cold air advection by Saturday will usher in the first true taste of winter to the lower elevations. 850 and 700 mb temperatures drop below the 1st percentile on the NAEFS situational awareness table, which is a strong signal at this range for anomalously low temperatures for mid fall. Highs in the 30s in the interior/higher terrain appears likely, while even to the coast temperatures likely only reach the 40s. Lows in the 20s-30s are likely, possibly to the coast. Temperatures look similarly chilly with less wind on Sunday, before beginning to rebound early next week. Upslope snow showers are likely in the mountains Saturday, and light accumulations are possible for the higher peaks.

One other headline for the Friday-Saturday timeframe will be potentially strong winds. Two likely bursts of wind: Friday ahead/along the trough of the S/SW, then Saturday behind the trough from the NW. The ECMWF has 850 mb winds around 35-45 kts for each burst, while the GFS is quite a bit stronger at 45-55 kts. Mixing should be much deeper for Saturday in the post-frontal airmass, so that is something to consider for surface wind potential. In any case there may be a window or two for advisory-level winds.

AVIATION /10Z TUESDAY THROUGH SATURDAY/

Through 12Z Wednesday...Conditions will continue to deteriorate this morning and afternoon as ceilings further lower and rain overspreads the region. Widespread IFR to LIFR restrictions are expected through the day and last to last through tonight. Rain is expected to end by 12Z Wednesday, but fog may develop after that. Also of interest: northeast winds could briefly gust to around 20 kt early this morning, and then a period of downsloping winds up to around 25 kt is possible at HIE and possibly LEB today.

Outlook:

Wednesday: MVFR to IFR ceilings possible in the morning, possibly fog and drizzle too. Most sites should return to VFR by the afternoon, except MVFR ceilings and showers remain possible at HIE. Northwest winds gusting 20-25 kt possible.

Wednesday night-Thursday: Mainly VFR, except MVFR remains possible at HIE. Northwest wind gusts to around 25 kt possible Thursday.

Friday-Friday night: Widespread IFR and rain are likely. Rain should taper off Friday night with improvement to MVFR or VFR. HIE may see lingering rain showers switch to snow showers with IFR or MVFR there.

Saturday: Rain and snow showers likely at HIE to bring MVFR/IFR. Elsewhere VFR, although it's possible a few showers make it downwind of the mountains. It also looks very breezy with west to northwest winds of 30 to 40 kt possible.

Sunday: VFR with lighter winds.

MARINE

SCA conditions are expected through at least Wednesday afternoon. East to northeast winds increase behind a backdoor front early this morning, and these will gradually veer southeast to south this evening as low pressure approaches. Winds and seas should briefly subside later Wednesday into Wednesday as the low lifts into the Canadian Maritimes, and then another period of SCA gusts is possible on Thursday as northwest winds increase as high pressure approaches.

A low pressure and strong cold front quickly approach from the west on Friday and cross later in the day into Friday night. Gales appear likely ahead of and behind this system, and then conditions should improvement toward the end of the weekend.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 2 PM EDT Wednesday for ANZ150-152- 154. Small Craft Advisory from 5 PM this afternoon to 2 AM EDT Wednesday for ANZ151-153.