Area Forecast Discussion
Issued by NWS Boston, MA
Issued by NWS Boston, MA
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756 FXUS61 KBOX 050622 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 222 AM EDT Fri Jul 5 2024 .SYNOPSIS... A few showers will impact the region into early this evening with the focus across the interior. A better chance for scattered showers and isolated thunderstorms will be overnight and at times Friday and Saturday. That being said...dry weather should still dominate.. Sunday though Monday is warm, humid, and dry. A return to more unsettled weather by Wednesday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 1000 PM Update: Considerably more humid than prior nights but other than varying amts of cloud cover, hasn`t been much in the way of showers after the earlier round exited. There are a cluster of heavier showers in scattered coverage from parts of eastern NY westward running along the NY/PA border. Really poor run-to-run handling in the hourly-updating CAMs as far as shower depictions go. Meteorologically, forcing for these showers looks nebulous frankly with weak sfc pressure gradient; but seems to be being driven by a warm front/theta-e boundary which trails from a weak low over central OH eastward into northern PA. This is seen on SPC mesoanlyses. Most models show PWAT axis in the 2 to 2.2 inch range advecting in for the 2nd half of the overnight. On that basis, it seems reasonable to offer increasing shower chances (20-40%, up from < 15%) for the overnight for the southern third - CT, RI, SE MA areas, with slight chance to dry north of the Mass Pike. Thinking until midnight-1 AM, much of the area is generally dry, but the risk increases thereafter from CT initially then spreading ENE into RI/eastern and southeast MA. Phrased as isolated/scattered showers for the late-overnight/early Fri AM period. With south winds easing, very weak low-level flow, and rising dewpoints, seems like a potential setup for fog/mist in places late tonight and overnight; challenging to pinpoint specifics but areas which saw rain today and where winds go light would be most favored. 405 PM Update... * A few showers into this evening...focused across the interior but may briefly impact the Boston to Providence corridor * Muggy tonight with the main risk for scattered showers & perhaps an isolated t-storm or two mainly after midnight Scattered showers have developed across southwest MA and northern CT as of mid-late afternoon. This in response to shortwave energy pushing eastward coupled with some marginal instability. We can not rule out a rumble or two of thunder across the interior into this evening...but instability and forcing is rather limited. Further east onto the coastal plain...just a few brief showers are expected into the evening. This a result of the shortwave energy weakening as it runs into subtle ridging. Appears the better risk for some scattered showers and perhaps a thunderstorm or two will be after midnight and especially toward daybreak. This in response to a subtle surface wave that will interact with very high Pwats on the order of 2-2.25 inches and several hundred J/KG of MUCape. Therefore...any of the activity will be capable of producing brief downpours. Because the forcing is limited but the Pwat plume is very high...the uncertainty revolves around the areal coverage of the activity and the location of it. This may need to be more of a nowcast situation...so later shifts will likely have to adjust the Pops/timing based on radar and satellite trends. Moist southerly flow will keep overnight low temps mainly in the upper 60s to the lower 70s and it will be muggy. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SATURDAY/... Key Points... * Very Warm & Humid Friday with highs 85-90 away from south coast * A few showers/isolated t-storms Fri with brief downpours possible Friday... A ridge of high pressure off the coast will result in a very warm and humid day on Friday. The majority of the day will likely feature dry weather...but a few showers/isolated t-storms will be possible at times given the tropical environment in place. We will discuss that below...but overall expect highs to reach between 85 and 90 Friday away from the south coast/Cape and Islands where highs will be in the upper 70s to the lower 80s. Scattered showers and perhaps a few t-storms will probably be departing the region Fri morning with the subtle shortwave. A few additional showers/isolated t-storms may popup later Fri morning and especially during the afternoon given a very high Pwat plume in place and any diurnal instability. The difficulty is trying to locate/time this activity given limited forcing in a tropical-like environment with Pwats of 2-2.25 inches. Models tend to struggle in these weakly forced environments...but it will not take much for a few showers/isolated t-storms to develop. Greatest concern would be during the Fri mid-late afternoon hours into the first part of Fri evening across the interior MA and CT. This is where surface Capes may approach 1500 J/KG and CSU machine learning probs along with HRRR Neural Network/Nadocast indicate some very low severe probs in our western zones. Again...forcing is limited but given the environment can not rule out the low risk for an isolated severe t- storm/localized wet microburst or two. In addition...a very localized flood threat can not be ruled out either given Pwats 3+ standard deviations above normal. So any t-storm that is able to develop will be capable of producing torrential rainfall. Friday night... Initially will have to watch for a few showers/isolated t-storms early Friday evening. This activity will probably start to dissipate though with the loss of diurnal heating coupled with poor mid level lapse rates. However...we may see scattered showers/isolated t- storms redevelop overnight given a modest southerly LLJ and very high Pwats on the order of 2 to 2.5 inches! While much of this time will feature dry weather...any activity will be capable of producing brief torrential rainfall given the tropical environment in place. The very high dewpoints will probably hold overnight low temps in the upper 60s to the middle 70s...so it will be quite muggy. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Key Points: * Mid-summer heat and humidity through the period * Potential for local heavy downpours Saturday and perhaps Wednesday Latest global ensemble model outputs don`t really show any big changes from prior forecasts. The large scale weather pattern indicates a mid/upper level ridge of high pressure will remain anchored to our east, with the ridge axis from roughly Bermuda north into the Canadian Maritimes. That puts us on the western periphery of the ridge, although 500mb heights will remain above normal. That kind of pattern would keep deep southwest flow across our area, meaning an extended period of high dewpoint air. It would also be hard to get any strong upper level dynamics to really impact the region as any shortwave would be moving into the ridge. What we will primarily experience as sensible weather for the area will be the humidity. NBM data looked quite good, so that would suggest dewpoints will rarely be below 65F through the period. Saturday and then again Tuesday and Wednesday should be the most humid of the days, with dewpoints in the 70-75F those days. Luckily we aren`t looking at extremely high temperatures (mid to upper 80s those 3 days), so heat index values will "only" be in the lower 90s. Those most humid days are also when we will have the higher chances of showers and thunderstorms. Given precipitable water values expected to be 2-2.25" on Saturday and again mid-week, that sets us up for the potential for locally heavy rainfall. Ensemble model probabilities show about a 20-30% chance of seeing 0.5" in a 6 hour period for Saturday and again Wednesday. Although those values don`t seem high, they are a respectable signal given the lower resolution of global models. With Saturday, surface CAPE values should reach 500-900 J/kg especially across western half of SNE so some convection should develop. Wind fields are not strong and mid level lapse rates are only 5-5.5C/km so not expecting any severe weather, but an isolated strong storm is not out of the question. As mentioned previously, it`s the high PW values that suggest local heavy downpours as the primary issue. Looks to be mostly dry Sunday and Monday, so those days will have the best potential to top out in the 90-92F range in many inland areas. Though it will be humid, dewpoints will be not be high enough to result in oppressive conditions. Another surge of higher moisture comes back in for Tuesday and Wednesday, thus additional showers and a few storms are expected. Much more uncertain for Thursday, so stuck with NBM which offers a climatologically normal 30-40% chance of showers and a few t-storms. However this time of year there are rarely strong synoptic signals to really latch onto, so sticking with a "climo" forecast is the best bet. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 06Z TAF Update: Though 12z Fri: Moderate confidence overall, but low on timing. Initially, mostly VFR but with pockets of MVFR-IFR ceilings/visbys around. Deterioration ancipiated to a general MVFR to IFR-LIFR from stratus and areas of fog, with lowest categories for PVD and southeast MA with reasonably high confidence. Categories along and north of the Mass Pike on the other hand are less certain and could stay in the VFR-MVFR range. Ongoing -SHRA/SHRA over CT to move eastward into RI-SE MA through the overnight/pre-dawn hrs and could briefly reduce visby to MVFR levels. Light S winds for most, though around 5-8 kts for BOS and the south coast. Today: Moderate confidence overall, but low on timing. Main aviation concerns this period revolve around timing improvement, if any, of stratus and then SCT SHRA/TSRA for central and western airports. MVFR-LIFR stratus is likely to begin this morning for most, and while we should start to see SCT-BKN VFR bases for most by 13-15z, sites such as PVD and the Cape airports could struggle to see much improvement. There looks to be a period of TSRA today in western New England as skies start to scatter, indicated PROB30 groups for TSRA after 18z for BAF, BDL and ORH, but confidence in the timing is lower than normal. Heavy donwpours likely in TSRA. SW winds today around 5-10 kt; it is possible that BOS could seabreeze for at least a brief period of time ~15-18z before returning to SWly by mid to late aftn. Tonight: Moderate confidence overall, but low on timing. Any TSRA for central/western airports to weaken thru 02z; dry weather should prevail for several hours, but we could see another round of SHRA/TS for these same areas later tonight/overnight. Deterioration toward a widespread IFR- LIFR tonight into overnight but timing uncertain. Light S winds. Saturday: Moderate confidence overall, but low on timing. IFR-LIFR generally improves to SCT-BKN VFR, but seems to be a better chance at TSRA on Sat especially ORH-PVD north and west. Timing of TSRA uncertain and subject to adjustment. SW winds 4-8 kt. KBOS TAF...Moderate confidence. VFR should deteriorate to MVFR cigs by pre-dawn hrs today but should see improvement quickly. Can`t rule out a shower early this morning but more likely dry. Possible seabreeze period ~15-18z but this isn`t certain. Aftn/evening SHRA/TS chances today too unlikely to mention in TAF, better chances further west. Categories deteriorate to IFR levels tonight. SW winds around 5-10 kt outside of a potential brief seabreeze. KBDL TAF...Moderate confidence. VFR should deteriorate to MVFR/IFR briefly 08-12z, before returning to SCT-BKN VFR bases by 13z. Early SHRA dissipates by 08z, with dry weather until aftn. PROB30 for TSRA after 18z and some could be strong with heavy downpours and localized wind gusts if storms develop. Risk should end by 02z but categories then deteriorate to IFR-LIFR with stratus. S winds 4-8 kt. Outlook /Saturday through Tuesday/... Saturday: MVFR/IFR conditions possible. Breezy. Chance SHRA, slight chance TSRA. Saturday Night: Mainly VFR, with local IFR possible. Breezy. Chance SHRA, isolated TSRA. Sunday through Monday Night: VFR. Tuesday: VFR. Slight chance SHRA. && .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Tonight through Friday night...High Confidence. High pressure east of the waters will generate persistent S-SW winds of 10 to 15 knots with perhaps some 20+ knot wind gusts at times. Conditions though should generally remain below small craft thresholds...but given long southwest fetch we may see some marginal 5 foot seas develop across our southern outer- waters Fri night. We may also have to watch for an isolated t-storm or two...but widespread convection is not expected. Outlook /Saturday Night through Tuesday/... Saturday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain showers. Saturday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers, isolated thunderstorms. Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Monday through Tuesday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank/Nash NEAR TERM...Frank/Loconto/KP SHORT TERM...Frank LONG TERM...Nash AVIATION...Loconto/Dooley MARINE...Loconto/Dooley