Area Forecast Discussion
Issued by NWS Boston, MA

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696 FXUS61 KBOX 181054 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 654 AM EDT Mon Jun 18 2018 .SYNOPSIS... Excessive heat and humidity is expected this afternoon away from the cooling marine influence of the south coast. A cold front approaching from the west may result in a few strong thunderstorms this afternoon and evening...mainly across interior southern New England. A much nicer day is on tap for Tuesday behind the cold front with plenty of sunshine and much lower humidity. Skirting wet-weather Wednesday followed by a period of seasonable, dry weather. Return of warmer, muggier conditions into the weekend with the chance of showers and thunderstorms. Perhaps a return of seasonable, dry conditions early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Highlights... * Excessive heat/humidity this afternoon northwest of I-95 * Isolated strong-severe t-storms with localized heavy rain possible between 2-10 pm mainly across interior MA/CT Details... 1) Excessive Heat/Humidity: Anomalous airmass was moving into southern New England early this morning. 850T should rise to between +18C and +20C along with plenty of sunshine into mid afternoon. This coupled with excellent mixing on gusty southwest winds should allow high temps to exceed guidance numbers in many locations. Highs should reach into the middle to upper 90s to the northwest of the I-95 corridor. Dewpoints rising well into the 60s should result in afternoon heat index values around 100 degrees and heat advisories are posted for this region. Meanwhile...southeast of the I-95 corridor there will be enough of a marine influence to hold highs in the upper 80s to near 90 with upper 70s to lower 80s on south coast beaches. In fact...gusty southwest winds will result in these high temperatures occurring by early afternoon and cooling marine air will be advancing northward later in the day. 2) Isolated Strong to Severe Weather Potential: There is uncertainty in regards to the convective potential in southern New England today....but there is the risk for isolated strong to severe thunderstorms mainly across interior MA/CT. Most of the high resolution guidance develops isolated to scattered convection after 2 or 3 pm across interior southern New England in association with a pre-frontal trough. We may see another round of scattered convection...perhaps with a bit more areal coverage after 7 or 8 pm in association with the surface cold front. The biggest limiting factors are weak mid level lapse rates which will limit updraft strength/potential and also uncertainty in regards to the amount of forcing. However...given the anomalous airmass in place there will be modest instability along with 0 to 6 km shear of 30 to 40 knots. This may be enough for isolated strong to severe thunderstorms mainly across interior MA/CT with the main threat being localized strong-damaging wind gusts. While the tornado threat is low it can not be completely ruled out either, because there is decent low level helicity. Pwats 3+ standard deviations above normal will result in the potential for any thunderstorm to produce torrential rain and a very localized street flooding threat. The strong to severe weather threat appears rather low across eastern MA/RI as forcing will be slower to arrive and a stabilizing marine layer will be working up from the south lowering instability. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... Late tonight... While the isolated severe threat should diminish by late evening...the potential for scattered showers and embedded t-storms will continue into much of the overnight hours. This a result of some elevated instability coupled with a cold front moving into a high Pwat environment. We should see the scattered shower/isolated t-storm threat confine itself to the south coast toward daybreak. Low temps will only fall back into the upper 60s to the lower 70s in most locations and it will be quite muggy for most of the night. Fog may impact the Cape/Islands as well given high dewpoint air in place. Tuesday... A much more comfortable day is in store for the region on Tuesday behind the cold front. Much drier air should work in from the northwest allowing for plenty of sunshine and significantly lower humidity. High temperatures should top out in the lower to middle 80s...but it will feel refreshing after today/s heat and humidity. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... */ Highlights... - Skirting S-coastal rains Sunday night - Cool, dry late-week pattern - Return of warm, muggy conditions into the weekend - Accompanying chances of showers and thunderstorms */ Overview... Back and forth. Rossby wave train evident. A sharpening Aleutian H5 trof, a present downstream ridge-trof-ridge pattern up against the Baffin Bay gyre. Midweek broadening, pattern breaks, flatter, shifts E, with Pacific energy rotating towards the gyre, S-stream energy at times drawn N, accompanying warm, muggy air. But with re-affirmation of the Aleutian trof, downstream pattern amplifies once again, air- masses transition. So a cooler, drier early week pattern beneath NW flow, accompanying H5 trof, energy drawn through amplified via cyclonic digging, parent cold front around Wednesday night ahead of which some rains could skirt S New England. Pattern broadening, higher heights, bit more S- stream dominance over Baffin Bay gyre, a warmer and muggier pattern towards the weekend. If the upstream Aleutian trof sharpens, down- stream pattern amplifying, our natural air conditioner could re- emerge towards late June, otherwise Summer is here. Targets of opportunity in the discussion below. */ Discussion... Around Wednesday night... Weak Mid-Atlantic energy emerging as H5 trof digs around the Baffin Bay gyre. Hesitant on PoPs given a weak low up against a dominant Canadian high. Fast, flat flow at the base with the N-stream vort- max digging, left feeling rain will skirt the S-shore, remain mostly offshore, S New England between the two waves, prevailing sinking air, less favorable environment. Keep chance PoPs confined to S waters, Islands. Thursday through Saturday... High pressure, likely sea-breezes. Transitioning over S New England Friday, monitoring radiational cooling, interior fog issues. Leaning low-end of MOS guidance for nighttime lows. High pressure E, return S flow ahead of warm front Sunday, marine stratus may return. Sunday into Monday... Lifting warm front followed by a weak cold frontal passage. Watching for showers and thunderstorms. Return of warm, muggy conditions. Too early on specifics, a low confidence forecast. && .AVIATION /12Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...Moderate confidence. Today... VFR through early afternoon. Low risk SCT MVFR-IFR along S-coast where SW winds will be gusting upwards of 35 kts. SCT SHRA / TSRA developing by 18z, 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, before winds shift NW by Tuesday morning. Tuesday... VFR conditions expected. KBOS Terminal... SHRA / TSRA into terminal roughly 21z. SCT nature, will prevail with VCTS. KBDL Terminal... SCT SHRA / TSRA into the terminal roughly 20z. Will prevail accordingly with VCTS. Outlook /Tuesday Night through Friday/...Moderate confidence. Tuesday: VFR. Breezy. Patchy FG. Tuesday Night: VFR. Wednesday: Breezy. Wednesday Night: Slight chance SHRA, slight chance TSRA. Thursday through Friday: && .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. Today... Southwest winds increase to between 20 and 30 knots by afternoon. Strongest of those winds will be near shore with excellent mixing given daytime heating...so very choppy waters to develop in Harbors and Bays. Long southwest fetch will also build sees to between 3 and 6 feet. 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 Friday/...Moderate confidence. Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft. Patchy fog. Tuesday Night through Wednesday: Winds less than 25 kt. Wednesday Night: Winds less than 25 kt. Slight chance of rain showers, slight chance of thunderstorms. Thursday through Friday: Winds less than 25 kt. && .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 from 1 PM this afternoon to 6 PM EDT this evening for CTZ002. Air Quality Alert from 11 AM this morning to 10 PM EDT this evening for CTZ002>004. MA...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for MAZ003>007-009>016-026. Heat Advisory from 1 PM this afternoon to 6 PM EDT this evening for MAZ003-005-006-010-011-013>016. RI...None. MARINE...Small Craft Advisory from 10 AM this morning to 10 PM EDT this evening for ANZ230>237-251. Small Craft Advisory from 11 AM this morning to 4 AM EDT Tuesday for ANZ250. Small Craft Advisory from 11 AM this morning to 10 AM EDT Tuesday for ANZ254>256. && $$ SYNOPSIS...Frank/Sipprell NEAR TERM...Frank/Sipprell SHORT TERM...Frank LONG TERM...Sipprell AVIATION...Frank/Sipprell MARINE...Frank/Sipprell CLIMATE...WFO BOX Staff

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