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Area Forecast Discussion
Issued by NWS Boston, MA
Issued by NWS Boston, MA
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922 FXUS61 KBOX 300622 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 222 AM EDT Sun Jun 30 2024 .SYNOPSIS... A slow-moving frontal system brings two rounds of scattered showers and thunderstorms, tonight into early Sunday morning, and then perhaps again Sunday afternoon and evening. Passing widely scattered afternoon showers are possible on Monday, although drier weather prevails most of the time. High pressure brings dry weather, seasonably warm temperatures and low levels of humidity for Tuesday and Wednesday. While the timing is still uncertain, turning more humid and unsettled for the Fourth of July thru the early part of the weekend with a couple of disturbances offering chances for clouds, showers and thunderstorms. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 2 AM Update... Forecast generally on track. Did make some minor adjustments to precip chances to better reflect current obs. Blended forecast with latest ARW/GDPS as they were doing fairly well based on latest observations. The result is better chances across portions of the interior, but am thinking risk really will increase along the south coast as dew points climb and the LLJ intensifies. Should have better opportunities for thunderstorms across the south coast, but cannot completely be ruled out across the interior. 735 PM Update: Radar imagery at this hour reflects what is amounting to very light showers largely north and west of I-95, gradually moving eastward. However METARs and mesonet information indicate little if any QPF at all with this, with only a handful or so of sites actually recording measurable precip. Did boost PoP up into the Likely/low Categorical range at least thru midnight for most areas, but at least through midnight, this is the textbook "high-PoP/low-QPF" scenario that you`ll ever find. So while radar looks imposing, there really isn`t much rain falling from these echoes. Also backed off on the thunder mention for this period of time. However we think that changes for the 2nd half of the evening into early Sun AM, but mainly for CT-central/southern RI-SE MA. Watching a cluster of t-storms with cooling cloud tops over eastern PA NW of the Philly area with additional re- development over north-central PA, with both areas moving ENE in the WSW flow aloft. For Southern New England, most models show rising dewpoints into the lower 70s later in the overnight into the pre-dawn hrs. With higher dewpoints at night, LCL heights are lower and with that comes somewhat higher most-unstable CAPE values (around 500-900 J/kg or so) mainly from HFD-IJD-PVD-PYM southward. Convective-permitting guidance varies on the handling of this, with the past several HRRR runs being the most bullish in the outlined area, with more tepid/muted answers from the WRF suite and the 18z NAM-3km. So for the 2nd half of the overnight into early Sunday, ended up reducing PoP elsewhere to around a chance level (25-30%) but boosted PoP to categorical for the southern third, added mention of thunder and increased QPF during the 06-12z Sunday period. Not expecting severe with this but if current/new development sustains itself in this setting, we could see overnight lightning and heavy downpours roughly from Hartford to Providence to Cape Cod. The rising dewpoints should also lead to more expansive stratus development for the coastal areas, but pretty strong southerly winds should keep fog to a minimum. No changes to lows which look on track attm. Previous discussion: Showers assocd with a weak mid level shortwave and anomalous PWAT axis are moving into northern and western MA this afternoon. Expect a period of showers across all SNE through the evening and into the overnight as deep moisture axis with 2-2.25" PWATs move into the region. Marginal elevated instability develops so can`t rule out an isolated t-storm, but the best chance may come toward daybreak along the south coast and Cape/Islands. A few of the CAMs are showing this potential but there is uncertainty regarding whether the convection stays offshore or extends further north along the south coast. Something that needs to monitored overnight as high PWAT airmass supports heavy rainfall. Otherwise, it will be a somewhat breezy night as modest low level jet develops. The SW flow will bring increasing humidity as dewpoints climb to around 70 overnight. Temps will be nearly steady in the upper 60s to lower 70s. Stratus and patchy fog will develop. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/... Sunday and Sunday night... * A round of strong to severe t-storms possible Sun afternoon and evening * Damaging wind gusts are the primary threat but large hail, heavy rainfall and also a tornado is possible An area of showers and a few t-storms may be ongoing along the south coast and Cape/Islands to start the day. There is lower than normal confidence as convection could end up further south but if it does extend along the south coast, it will move offshore by late morning. Otherwise, stratus and patchy fog will burn off away from the south coast leading to partial sunshine. Given warm sector airmass with 925 mb temps 22-24C, temps should reach well into the 80s to near 90 away in the interior. Stratus will likely linger into the afternoon along the south coast with temps holding in the upper 70s with SW flow. Oppressive humidity is likely with dewpoints reaching the low to mid 70s. Heat indices expected to approach 95 in the CT and Merrimack valleys. The main forecast concern will be the potential for strong to severe t-storms in the afternoon and evening as a strong cold approaches from the west. The increasing forcing for ascent will likely result in a greater areal coverage of storms. Mid level lapse rates are marginal, but the heat and high dewpoints should make up for this with CAPES likely reaching 1500-2500 J/kg in the interior. This instability combined with impressive deep layer shear of 40-50 kt is very favorable for organized storms. Given strong linear forcing ahead of the front, storms will likely organize into line segments or even some sort of QLCS as indicated by all the hi-res CAMs. Damaging wind gusts will be the primary threat, but can`t rule out large hail or even a tornado if any discrete supercells can develop. SPC SREF showing moderate probs of STP > 1. Best chance of severe weather will likely be north and west of I-95 due to marine layer near the coast and CSU machine learning probs and HREF updraft helicity are highlighting the interior for severe weather. Also can`t rule out locally heavy rainfall and flooding from t-storms but severe weather appears to be the greater threat. Storms are expected after 2 pm across interior northern and western MA, gradually reaching the south coast Sun evening. Convection should be in a weakened state by the time it reaches the coast due to weaker instability but strong shear may help to sustain convection a bit longer. Showers and a few storms could linger into the overnight period over the Cape/Islands, otherwise decent drying will be moving into the region overnight as cold front gradually pushes offshore with NW flow developing. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Highlights: * Hit-or-miss widely scattered showers Mon, but most stay dry with lowering humidity levels. * Dry and warm but with tolerable humidity levels for Tue and Wed. * Turning more humid and more active for the Fourth of July, and potentially into Fri/Sat too, with clouds, showers and storms possible. Exact timing still uncertain. Details: Monday: Surface ridge associated with a seasonably-strong 1024 mb high pressure over the Gt Lakes region eventually builds in. Will still see generally improving conditions, although shortwave disturbance aloft and associated cooler pool of air aloft should produce enough if still meager instability to pop diurnal cumulus clouds and hit-or- miss showers in widely scattered coverage. Can`t really rule out a shower anywhere in Southern New England, although central MA eastward has somewhat better chances with somewhat higher surface dewpoints (lower 60s). However not expecting washouts with these showers. Conditions improve significantly toward drier weather by the evening both with waning diurnal instability and passage of the shortwave disturbance aloft, with falling dewpoints into the 50s thru evening on northerly winds. Highs in the mid/upper 70s and lows in the mid/upper 50s. Tuesday and Wednesday: 500 mb heights briefly rise in this period, along with high pressure settles in over the Northeast in this period. This high shifts offshore later into Wed night. Leads to a couple of tranquil and pleasant early-July days with comfortable humidity levels and highs in the low to mid 80s. As high pressure moves offshore late Wed evening, SW flow and weakening 500 mb heights (upper ridge shifting east) should bring increased cloud cover and rising dewpoints/humidity levels. Flow aloft also strengthens a bit; ahead of a frontal system for the Fourth of July, that preceding stronger flow aloft could bring a canopy of cloud cover in potentially quicker than current forecast calls for on Wed evening. Thus lows also warmer in the mid to upper 60s. Fourth of July Holiday: While primary weak-amplitude disturbance aloft in fast westerly flow passes to the north into Quebec/northern New England, the Fourth of July is looking active as a sagging cold front moves southeast through New England. This front should act on an increasingly humid air mass to favor showers and t-storms. Greatest uncertainties at this time lie in the timing and how warm/degree of instability, and kept PoPs pretty broadly in the Chance range for now and then re- assess once there is better clarity on each of those. But as it looks now it does look like Thurs and potentially into Thurs night features clouds and showers/storms. With dewpoints in the upper 60s to mid 70s, it should also be quite humid. Kept highs in the mid 80s for now as well, although clouds and shower/storm coverage will dictate adjustments to highs as well. Lows in the mid to upper 60s. Friday and Saturday: Forecast for Fri is uncertain as there looks to be another disturbance aloft which tries to send a warm front back into Southern New England either Fri or Fri evening/Sat. GFS is the quickest in progressing this feature into our area into Fri, while the ECMWF/Canadian GEM and its ensembles are slower, more into Sat. Kept PoP pretty broadly into the Chance range here given the uncertainties which are close to NBM values; these too will also need to be adjusted pending trends in subsequent guidance. && .AVIATION /06Z SUNDAY THROUGH THURSDAY/... Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Through 12Z...Moderate confidence. Will have MVFR to LIFR conditions with LIFR most likely across the immediate south coast. Most of the showers are not impacting visibility. Think that activity across interior will remain generally as showers/weaken, but cannot completely rule a few rumbles of thunder. Best shot for thunder comes across the immediate south coast as we head toward daybreak as dewpoints climb and the LLJ intensifies. Have tried to highlight this for PVD-FMH-HYA and ACK with a VCTS. Winds out of the S to SW at 10-15 kts with gusts of 20-30 kts. Today...Moderate confidence. Conditions improve to VFR for most by roughly 13-16Z. The only exception is for PVD-OWD where MVFR to LIFR stratus lingers. May have a few showers/storms exiting the Cape and Islands during this timeframe. Another round of showers/storms expected to materialize roughly 17-19Z across the interior. These will push southeastward toward the south coast by the evening. Strong to severe storms possible, which if impacting a terminal would bring the risk of damaging wind gusts and conditions briefly deteriorating to MVFR to LIFR. Have handled this risk with a PROB30 for all terminals, except across the south coast where am uncertain on how quickly the line weakens given the stable boundary layer as the marine layer will be in place. For now utilized a VCTS for this. Will have SW winds at 101-5 kts with gusts of 20-30 kts. Winds shifting to the W/WNW toward the evening across the interior. Tonight...Moderate confidence. MVFR to IFR with showers/t-storms in the evening along the south coast, then improving to VFR overnight. Wind shift to NW 00-06Z. Speeds of 5-10 kts with gusts diminishing toward 03-06Z, but will then increase to 15-20 kts toward daybreak. Monday...High confidence. VFR to start, but as CU develops will have borderline MVFR/VFR cloud bases. Should see hit or miss showers/storms developing, which could potentially produce some small hail. Winds out of the NNW to N at 5-10 kts with gusts of 15-20 kts. KBOS TAF...Moderate confidence in TAF. Conditions MVFR to IFR through roughly 12Z. Could see a spot shower heading into the AM push and perhaps a rumble of thunder. Though confidence in the thunder was too low to include a mention at this point. Will improve to VFR roughly 15-17Z. Scattered showers/storms developing 19-23Z and could bring strong to perhaps damaging winds and brief heavy downpours. Improving to VFR as showers/storms move toward the south coast by the evening. S to SW winds at 10-15 kts with 20-25 kt gusts. Winds shifting to the W/WNW during the evening once the showers/storms slide through. KBDL TAF...Moderate confidence in TAF. MVFR to IFR conditions as we head into the AM push today. Should gradually improve to VFR by 14-16Z. Cannot rule out a spot shower. Will have increasing chances of showers/storms this afternoon/evening, some of which could bring strong to perhaps damaging wind gusts. Have handled with a PROB30 from 17-23Z. These storms moves S/SE of the terminal during the evening and will fairly quickly improve to VFR. Winds out of the S at 5-15 kts. Will shift to the W/WNW this evening in wake of shower/storm activity. Could have some 15-20 kt gusts this afternoon (not associated with the thunderstorms). Outlook /Monday Night through Thursday/... Monday Night through Tuesday Night: VFR. Wednesday: VFR. Breezy. Wednesday Night: VFR. Breezy. Chance SHRA. Independence Day: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA, slight chance TSRA. && .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Through Sunday night...High confidence. Gusty S winds tonight becoming SW on Sunday with gusts to 25 kt at times. SCA continued for all waters. Winds shift to NW overnight Sun night behind the cold front. Vsbys reduced in developing showers and fog tonight, with fog lingering on Sunday especially south coastal waters. A few t-storms possible over southern waters late tonight and Sun morning, then more showers and t-storms moving across the waters late Sun and Sun night. Outlook /Monday Night through Thursday/... Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday through Wednesday: Winds less than 25 kt. Wednesday Night: Winds less than 25 kt. Slight chance of rain showers. Independence Day: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers, isolated thunderstorms. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 10 AM EDT this morning for ANZ230- 236-251. Small Craft Advisory until 8 PM EDT this evening for ANZ231- 250. Small Craft Advisory until 11 PM EDT this evening for ANZ232>235-237. Small Craft Advisory until 6 AM EDT Monday for ANZ254>256. && $$ SYNOPSIS...Belk/BL NEAR TERM...BL SHORT TERM...BL LONG TERM...Belk AVIATION...Belk/BL MARINE...Belk/BL