textproduct: Boston / Norton

WHAT HAS CHANGED

Monitoring late tonight into early Tuesday for snow showers. Otherwise, dry weather with increasing southerly winds Wednesday and Thursday could lead to fire weather concerns.

KEY MESSAGES

- Weak system moves through tonight into Tuesday bringing light rain/snow showers. No accumulation expected.

- Gradual warming trend starts Wed and Thurs, although it will be quite dry to go along with increasing south winds on Thurs.

- Mild and dry Friday heralds a cold frontal passage Sat but with limited rain chances. Onshore flow/possible backdoor frontal boundary then ushers in a cooldown for Sun.

DISCUSSION

KEY MESSAGE 1...Weak system moves through tonight into Tuesday bringing light rain/snow showers. No accumulation expected.

A deeper upper-level trough shifts across the region tonight and Tuesday. This trough brings along a cold pool aloft with 500mb temperatures around -30C and 925mb temperatures -3C to 1C. A shortwave trough moves through the flow aloft supporting some weak lift and marginal moisture and even a little instability to support scattered "popcorn" showers. Considering that we have such cool temperatures aloft, this should support light snow showers for the higher elevations and snow mixing in elsewhere. Given weak convective element, can't rule out even a little graupel in any showers. High-resolution guidance is still lacking agreement on the finer details of the timing and shower coverage. Overall there is a signal for a few showers very early Tuesday AM. Probabilities increase (30-45%) for scattered showers closer to 7/8AM and after as the main surface low tracks across the region. Any snow/frozen precip accumulation would be very light and little impact. Worst case 1". Temperatures will be below normal in the 40s.

KEY MESSAGE 2...Gradual warming trend starts Wed and Thurs, although it will be quite dry to go along with increasing south winds on Thurs.

Rather strong 1038+ mb high pressure cell south of Nova Scotia early Wed will extend a surface ridge southwestward through the Southern New England coastline into the mid-Atlantic and central Appalachians, remaining more or less in place into Thurs. Warming 925 to 850 mb temps and full sunshine should permit a general warming trend thru Thurs with highs on Wed in the mid 40s to low 50s (low 40s near the coast) and into the 50s in most areas by Thurs. With a few days of generally dry weather early this week, fire weather concerns are possible on both days, as dewpoints drop into the teens to low 20s in the pre-greenup period allowing for minimum RH's in the 20 to 25 percent range from the coast. Although southerly winds are on the light side Wed, they pick up a little more on Thurs with gusts around 20-25 mph, so Thurs probably offers the strongest chance for fire weather concerns.

KEY MESSAGE 3...Mild and dry Friday heralds a cold frontal passage Sat but with limited rain chances. Onshore flow/possible backdoor frontal boundary then ushers in a cooldown for Sun.

Looks like one final day of full sun and even milder temps back into the mid 60s Friday, so it looks like Fri ends up as the pick of the workweek. Sat starts off mostly sunny, but will have increasing cloud cover as a weakening frontal system moves through Southern New England during the afternoon to first part of the evening. This frontal boundary won't have much moisture to work with, and it may just result in some enhanced cloudiness with limited prospects for rain showers. Looks like late Sat night or into Sunday, guidance continues to show presence of a NE onshore flow/possible backdoor front with cool 850 mb temps 0 to -2C. It's still early but it doesn't look like there is a signficant amount of RH and pressure gradient doesn't look too strong, so this backdoor front doesn't look to be as pronounced in terms of strong onshore winds and as overcast as e.g. this past Saturday's backdoor front was. Regardless, expect a cooldown as we move into the latter part of the weekend, though temperatures start to modify once again as we move into early next week.

AVIATION /19Z MONDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

18 TAF Update...

Today: High confidence.

VFR. WNW winds 10-20 kts and gusts 20-30 kt. Highest gusts for higher elevations and Cape/Island terminals.

Tonight: High confidence overall, moderate on possible precip chances.

SCT-OVC cloud decks return overnight, VFR mostly. There is still uncertainty on the timing and coverage/chances of showers tonight. There is potential for brief -SHRASN after 06Z with pockets of brief MVFR possible. Gusts diminish by 01Z with W winds less than 10 kts. Winds become light and variable after 06Z.

Tuesday: Moderate confidence.

VFR with periods of MVFR ceilings improving to VFR mid afternoon/early evening. Scattered, brief -RASN showers Tuesday AM through early afternoon. Timing and coveraging lower confidence. Winds will shift through the day as a surface low passes through. Light SE flow to start, eventually shifting to NW from west to east in the afternoon. NW winds will trend stronger 10-15 kts with gusts around 20 kts.

Tuesday Night: High confidence.

VFR conditions. NW winds 8-12 kts, a few gusts up to 20 kts possible.

KBOS Terminal...Moderate Confidence in TAF.

Few -RASN showers early after 09Z through 12Z, messaged as a PROB30 due to uncertainty in model guidance and low coverage of SH. Probabilities for scattered showers increase more toward 12Z, so included in TAF for -SHRASN through the afternoon. Showers will likely be brief in nature with MVFR ceilings. SE winds in the early AM shift to E/ENE by noon, shifting N late in afternoon.

KBDL Terminal...High Confidence in TAF.

Outlook /Tuesday Night through Saturday/...

Wednesday through Wednesday Night: VFR.

Thursday through Thursday Night: VFR. Breezy.

Friday: VFR.

Friday Night: VFR. Slight chance SHRA.

Saturday: VFR.

MARINE

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

High confidence.

Gusty winds diminish this evening with seas 2-4 ft. This will allow the small craft advisories to be dropped this evening.

Low pressure moves across region Tuesday with winds shifting through the day from SE,eventually around to the N/NW late afternoon-early evening. Northwesterly wind gusts may approach SCA levels Tuesday evening across the southern waters ahead of an approaching high pressure system.

Outlook /Tuesday Night through Saturday/...

Wednesday: Winds less than 25 kt. Seas locally approaching 5 ft.

Wednesday Night through Thursday Night: Winds less than 25 kt.

Friday: Winds less than 25 kt. Seas locally approaching 5 ft.

Friday Night through Saturday: Winds less than 25 kt. Seas up to 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ254>256.