Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 182011 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 411 PM EDT Mon Jun 18 2018 .SYNOPSIS... Heat and humidity will come to an end tonight as a cold front from the northwest pushes through. A few strong thunderstorms are possible especially across western MA and northern CT. A much nicer day is on tap for Tuesday behind the cold front with plenty of sunshine and much lower humidity. Low pressure passes off the South Coast Wednesday, with a chance of showers along the coast. High pressure from Canada brings dry weather Thursday and Friday. a cold front them brings a chance of showers over the weekend, followed by dry weather early next week. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... This afternoon and first half of the night... Heat...Southern New England remains on the edge of the upper level ridge anchored over the Great Lakes. This has resulted in any cloud debris from earlier convection to ride along the ridge and right over southern New England. These clouds have been quite thick especially during the heating of the day. Thus forecasted high temps have not been realized, esp across eastern MA. Went ahead and knocked them down a few degrees earlier today, and have brought it more in line to ongoing conditions. Dewpoints continue to rise this afternoon with several sites now 70F. With the warm temps and increasing dewpoints we have seen heat index values in the low to upper 90s across the CT River Valley, and lower Merrimack. Went ahead and dropped Norfolk and Suffolk counties as they have been just shy and the clouds will limit any more heating. Have seen a few 100F heat indices across BDL and BAF so will keep heat advisory in that region for the rest of the day. Will leave it for portions of Middlesex and Essex as there values are on the edge. Convection...Showers and thunderstorms continue to fire up across northern New England and upstate NY this afternoon. Better moisture and lift are still just outside of the region as of 300 PM. Per SPC meso-analysis the higher K values are starting to push into the region and with approaching pre frontal trough in MSAS MSLP should start seeing a few storms to pop-up here in the next two hours. Overall threat for severe weather still seems a bit marginal as the environment is not favorable. Mid-level lapse rates are still poor with just 5-5.5 C/km over the region with 0-6km shear near 25-35 kts. Cape values have increased to over 2000 J/kg thanks to the high dewpoints, but these values will only last for a few more hours as CIN will begin to mix into the atmosphere and we lose heating of the day. Still have to watch for the occasional strong to severe storm as directional shear is pretty good per BUFKIT sounding profile. Best location for any severe weather will be across western MA and northern CT which had the best heating today and highest probs from HREF, HRRR and SREF. Gusty winds and heavy rainfall are still the main threats for today. Cannot rule out a microburst potential, but believe that the low level lapse rates marginal (7.5 C/km) and DCAPE above 1000 J/kg. Heavy rainfall is most likely as PWAT values are over 1.5-1.8 inches and will continue to increase into tonight. Late Tonight... Approaching cold front from the north will swing through the overnight hours. Guidance continues to indicate that severe potential will be less as the atmosphere will struggle to recuperate due to ongoing/upcoming convection. However still cannot rule out a rumble of thunder overnight as K values are still high and showalters drop to be 0C. Still some lingering MU Cape, combined with the high PWATs, warm cloud processes and increasing LLJ, may have to watch for heavy rainfall. Especially as the dewpoints continue to increase overnight and pool up against the front. Precip should wind down after 2-5 AM due to FROPA. Still expect a muggy night on tap as the true dry air wont work its way into the region until Tuesday. Aside from the precip, will have to watch for fog and stratus across the south coast. Persistent south/southwest flow will continue overnight and with high dewpoints over the cooler waters could develop areas of fog. This is supported by the NARRE-TL and HRRR. Best region for low vsbys will be the Cape and Islands, as well as the immediate south coast. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... Tomorrow... A few lingering showers early Tuesday as a cold front finally pushes offshore. Dewpoints and K values will be quickly dropping in the late morning into afternoon and thus expect clearing skies and dry weather. Northwest winds will be gusty in the morning before high pressure moves into the area. Gusts to near 20-25 MPH are possible. Otherwise a pleasant weather day. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... Big Picture... Longwave pattern shows above normal heights across the western and southern USA with a persistant trough over the Great Lakes and Northeast USA. Northern stream jet flows from far northern Canada southeast across New England eventually moving north of us by the weekend. A shortwave embedded in this flow moves across New England Wed night or Thursday. A weaker southern flow across the central USA carries a closed low from the Northern Rockies to the Western Great Lakes. The low then ejects into the northern stream and crosses New England during the weekend. Mass fields over the Eastern USA are similar through Saturday, then diverge. Thermal fields agree on cooler temps Thursday-Friday followed by warming temps over the weekend, then trending cooler again early next week. Forecast confidence is high through Friday, trending to moderate over the weekend. Concerns... Tuesday night-Wednesday... High pressure builds over the region Tuesday night, then shifts offshore Wednesday. Northern stream shortwave dives southeast into New England. This shortwave will be supported by a 105 knot jet and drive a surface cold front toward our area. The jet may also provide enough upper divergence and lift to support a developing low along the Mid Atlantic coast late in the day Wednesday or Wednesday night. W values climb to 1.5 inches along the South Coast Wednesday evening before drier air pushes south across New England overnight. Expect dry weather Tuesday night and Wednesday. The Mid Atlantic system mostly passes offshore, but the upper lift may allow for rain to graze the South Coast. Thursday-Friday... High pressure builds over the region with dry weather and cooler temperatures. Saturday through Monday... Midwest upper low and associated surface system eject into the Northern stream by Saturday, then cross New England Sunday. Rain out ahead of the system arrives Saturday, with rain along the cold front Sunday afternoon or night. PW values increase again, with 2 inch values possible Saturday night and Sunday. && .AVIATION /20Z MONDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...Moderate confidence. Before 00z... VFR through early afternoon. Low risk SCT MVFR- IFR along S-coast in fog/stratus. SCT SHRA / TSRA developing by 20z, mainly interior MA / CT. RA/+RA possible along with IFR CIGs and VSBYs. W G30-40 kts possible with storms as well. Tonight... SCT SHRA / TSRA dissipating, as late as 3z, again mainly interior MA / CT. Continued RA/+RA threats along with IFR CIGs and VSBYs. Otherwise VFR. SW winds through early morning, LLWS for S-coast along with MVFR-LIFR CIGs and VSBYs in fog and stratus, before winds shift NW by Tuesday morning. Tuesday... VFR conditions expected. Breezy out of the NW. KBOS Terminal... SHRA / TSRA into terminal roughly 22z. Showers thereafter. KBDL Terminal... SCT SHRA / TSRA into the terminal roughly 21z. Showers thereafter. Outlook /Tuesday Night through Saturday/...Moderate confidence. Tuesday Night through Wednesday: VFR. Wednesday Night: Mainly VFR, with local MVFR possible. Slight chance SHRA. Thursday through Friday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...High confidence. Tonight... Near shore southwest wind gusts should diminish a bit by late evening...but marginal small craft wind gusts and 3 to 6 foot seas should persist longer over the open waters. An isolated t-storm or two is possible tonight. Tuesday... Lingering small craft swell across our southern most outer-waters diminishes by late morning. Otherwise...winds/seas should remain below small craft advisory thresholds. Outlook /Tuesday Night through Saturday/...Moderate confidence. Tuesday Night: Winds less than 25 kt. Seas less than 5 feet. Local visibility 1 to 3 nm in patchy fog. Wednesday: Winds less than 25 kt. Seas less than 5 feet. Wednesday Night: Winds less than 25 kt. Seas less than 5 feet. Chance of rain showers. Thursday: Winds less than 25 kt. Seas less than 5 feet. Slight chance of rain showers. Thursday Night through Friday: Winds less than 25 kt. Seas less than 5 feet. && .CLIMATE... Here are the record and forecast highs for Monday June 18... BOS ... 94 (1907 & 1929) ... Forecast High 95 BDL ... 95 (1957 & 1994) ... Forecast High 96 PVD ... 94 (1929) ... Forecast High 88 ORH ... 93 (1907 & 1929) ... Forecast High 91 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Heat Advisory until 6 PM EDT this evening for CTZ002. Air Quality Alert until 10 PM EDT this evening for CTZ002>004. MA...Air Quality Alert until 11 PM EDT this evening for MAZ003>007- 009>016-026. Heat Advisory until 6 PM EDT this evening for MAZ003-005-006- 010-011. RI...None. MARINE...Small Craft Advisory until 10 PM EDT this evening for ANZ230>237-251. Small Craft Advisory until 4 AM EDT Tuesday for ANZ250. Small Craft Advisory until 10 AM EDT Tuesday for ANZ254>256. && $$ SYNOPSIS...WTB/Dunten NEAR TERM...Dunten SHORT TERM...Dunten LONG TERM...WTB AVIATION...WTB/Dunten MARINE...WTB/Dunten CLIMATE...Staff is the U.S. government's official web portal to all federal, state and local government web resources and services.