410 FXUS61 KBOX 121103 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 703 AM EDT Thu Mar 12 2020 .SYNOPSIS... High pressure builds into the Gulf of Maine today and into the Maritimes tonight. A frontal system will bring a period of steady rain to Southern New England overnight into early Friday. Turning cooler and blustery Friday afternoon behind the cold front with rain ending. Dry and cool weather is forecast for this weekend. Another weather system brings rain showers, and possibly an initial coating of snow, late Monday night into Tuesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... No major changes to the forecast this morning. Dry weather through most of today with light NE to SE winds. Mainly brought the forecast for this morning back in line with observed trends. Previous Discussion... Early this morning, a 1026 mb high pressure area over the mouth of the St. Lawrence River in eastern Quebec was extending a sfc ridge through ME/NH into Southern New England. This is leading to light N to NE surface winds. With skies having largely cleared overnight we do have some significant variation in current temps. The urban areas were generally mildest this morning with mid/upper 30s, though temperatures were running generally near to below freezing in the outlying towns. Increasing moisture over the southern waters was also being advected northward per nighttime microphysics imagery on light E flow was leading to some lower clouds towards southern CT/RI early this morning. Aforementioned surface ridge builds SE into the Gulf of Maine through today. This will lead to continued dry conditions today, while an increasing pressure gradient especially late in the day should lead to a modest uptick in wind speeds. This will lead to an onshore flow component with N/NE winds to become E and eventually ESE late in the day. Some warming is anticipated even as clouds increase again as the day progresses. However, a modified diurnal temperature trend looks warranted for today given that the onshore flow will draw cooler temperatures (e.g. sea-surface temperatures in the low-mid 40s across Mass Bay into the Gulf of Maine) landward. Coolest high temperatures today to be confined to the eastern/southern coasts (mid 40s), while highs further in the interior should reach well into the 40s to low 50s by mid-aftn before hovering/cooling towards sundown. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tonight: The first part of the evening will feature similar conditions to late today, with strengthening pressure gradient between the aforementioned high over the Maritimes and a rather potent frontal system that starts to affect our area very late tonight and more so into the first part of Friday. Strengthening gradient should lead to SE winds around 10 mph, a bit stronger towards the Cape by midnight. Will also see increasing chances for showers across the CT Valley and into the eastern slopes of the Berkshires, with lowering clouds, though most stay dry thru midnight across the coastal plain, central MA into RI and the Cape. Overnight and especially towards early-morning, the rain potential stands to increase as a cold front moves into the Hudson Valley. By the pre-dawn hours, look for rain to be the steadiest mainly from the I-91 corridor westward and may be somewhat enhanced by upslope against the east slopes of the hills given the low-level SE flow. Increasing PoPs toward Chance/lower Likely levels towards the coastlines. Looking at cooling temperatures through about midnight before temperatures then start to rise with increasing warm and moisture advection. Lows mainly mid 30s to near 40, with low- mid 40s by sunrise. Friday: Friday looks pretty active, especially the first half of the day as the cold front progresses rather quickly eastward from eastern NY to the outer eastern waters thru early afternoon. CAM simulated reflectivity progs are pretty robust in indicating a solid Actually looks to be fairly stout frontal convergence acting on an increasingly moist air mass (PWAT values 1-1.25" per 21z SREF mean). There's also some shallow elevated instability indicated in some guidance toward central CT into RI and on into the South Coast and the Cape (e.g. 00z HREF MUCAPEs around 100-200 J/kg). The 00z HREF actually shows really strong neighborhood probabilities for 40 dbZ echoes in these same- described areas. Given these indications, I think it is prudent to at least mention the threat of isolated thunder for the Hartford-Providence corridor into SE MA. While this rain will be welcomed given protracted dryness, it will also be falling steadiest around or just after the morning commute. As such, it could make the Friday morning drive a bit more difficult, but progressive nature to the frontal zone should limit storm-total precip to around a half to three quarters of an inch. Pre- frontal SE low-level winds are quite strong (NAM/GFS 950 mb winds in the 40-50 kt range). These winds look to be locked up above the inversion and not likely to be transported to the surface, though may be possible if thunder materializes. Dry slot races in quickly for the afternoon with a wind shift from SE to W, rapid cloud clearing and sharp cold advection behind the cold front. In fact I think winds should prove to be gustiest in the post-frontal air, with gusts around 30 mph. Temperatures should warm to highs into the 50s just behind the cold front, but then fall back into the 40s to near 50 by early evening. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Mid level flow is projected to remain fairly stable into early next week. Pattern features a split flow merging into a confluent flow over the eastern USA. More differences arise towards the middle of next week. At the surface, the core of a large, potent high pressure will traverse Canada, reaching the Saint Lawrence valley and the Maritimes towards Monday. This will mean cooler and drier air later this weekend into early next week. By Sunday into Monday, temperatures should be near to slightly below normal. Once this high pressure moves past our region Monday night, temperatures will start to rise above normal once again. Before this happens, there is the small possibility for some some snow at the onset of some showers Monday night into Tuesday morning. This assumes the current timing pans out, which is not a given with a Day 5 forecast. Any snow should not survive much past daybreak in any case. High pressure builds over the Eastern USA Wednesday. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today: Moderate confidence. Mainly VFR with initial mid-cloud ceilings, though ceilings will be steadily lowering this afternoon as ESE flow strengthens a bit. A possibility that low-VFR to MVFR ceilings could occur into the morning at PVD, but may stay closer to coastal RI/CT. Similar potential exists this afternoon across NE MA and into BOS. Will definitely need to monitor that closely today. Winds to become E to ESE/SE as the day progresses, with an increase in speed to around 6-9 kt. Tonight: High confidence thru midnight, then trends moderate. BKN-OVC VFR ceilings to prevail through about midnight. Deteriorating conditions then expected for the second half of the night with ceilings lowering to sub-VFR levels for most sites except the Cape. Exact timing still somewhat unclear, but looking at 08-12z for degraded ceilings - MVFR coastal plain, and MVFR/IFR interior. VCSH indicated from ORH/PVD east, with steadiest frontal showers being restricted to BAF and BDL thru 12z. Winds ESE/SE 6-10 kt thru midnight, then become SE around 8-12 kt by daybreak. Strengthening SE low-level jet could result in pockets of wind shear ~08-12z across RI, South Coast into the Cape as well as in the high terrain. Friday: Moderate confidence. MVFR-IFR ceilings with MVFR-vsby steady frontal -RA/RA moving E thru 18z, with areas LLWS in that period of time. Outside shot at a rumble of thunder towards the South Coast and the Cape. Rapid clearing to VFR expected by afternoon as rain shifts into the offshore waters, with a gusty wind shift to SW/W. SE winds around 10-15 kt, near 20 kt to the Cape/ACK thru ~18z with areas of low-level wind shear. Winds to then shift to SW/W around 8-12 kt with gusts 20-28 kt, up to 35 kt towards ACK. in post- frontal clearing. KBOS Terminal...High confidence in TAF. Mention of BKN040 to account for possible lower ceilings toward late afternoon and evening. KBDL Terminal...High confidence in TAF. Outlook /Friday Night through Monday/... Friday Night: VFR. Windy with local gusts up to 35 kt. Saturday through Sunday: VFR. Breezy. Sunday Night through Monday: VFR. && .MARINE... While generally quiet weather conditions exist today, a Gale Watch has been hoisted for late tonight through early Friday evening for most areas except Boston Harbor, Narragansett Bay into Buzzards Bay. Highest confidence of Gale conditions on the eastern outer waters Friday, though may get close to Gale-force gusts on the southern waters and into Mass/Cape Cod Bays. Today...NE to E winds to trend ESE late in the day, around 10 kt with gusts around 15 kt. Seas 1-3 ft. Tonight...SE winds increase to 15-25 kt, with gusts to 25-30 kt on the southern and eastern waters. Seas build to 4-7 ft with 7-9 ft becoming increasingly rough toward the early-AM. Friday...SE winds to gale force at least through mid-aftn. Wind shift to SW/W 10-20 kt with gusts to 25 kt then thru early evening. Rough seas 8-12 ft on the northeast waters and 5-8 ft on the southern waters. A decrease in seas expected across southern waters late in the day. Visibilities 3-5 SM in rain, with isolated thunder possible towards the southern waters. Outlook /Friday Night through Monday/... Friday Night through Saturday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Sunday through Sunday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Monday: Winds less than 25 kt. Seas locally approaching 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Watch from late tonight through Friday evening for ANZ231>233-235-237-250-251-254>256. && $$ SYNOPSIS...Belk/Loconto NEAR TERM...Belk/Loconto SHORT TERM...Loconto LONG TERM...Belk AVIATION...Belk/Loconto MARINE...Belk/Loconto