National Weather Service (2024)

Area Forecast Discussion
Issued by NWS Portland, ME

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-- Discussion containing changed information from previous version are highlighted. --770FXUS61 KGYX 050224AFDGYXArea Forecast DiscussionNational Weather Service Gray ME1024 PM EDT Tue Jun 4 2024.SYNOPSIS...Some showers are possible through this evening as high pressuresinks south of the region. Chances for showers andthunderstorms increase Wednesday as a trough approaches from thewest. These showers will be slow moving and pose an isolatedflash flood risk. This trough likely brings periods ofwidespread rainfall late Thursday which also may pose anisolated flood risk. Unsettled conditions persist through theend of the week with daily chances of showers and an isolatedafternoon thunderstorm.&&.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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Update...Any precipitation in the form of showers and thunderstorms willremain isolated and locked in over eastern New York andsouthern Vermont. Varying amounts of clouds will be placed overour forecast area overnight. Patchy fog has already begun toform and will become more persistent after midnight. Otherwise,just minor adjustments to the ongoing temperature and dew pointtrends for the near term update.Prev Disc...A few isolated showers continue in the mountains and foothillsearly this evening per latest radar imagery and surface reports.Have expanded the pops into central portions of Maine and NewHampshire but the overall trend will be for this precipitationto diminish as we head into sunset.Patchy fog expected once again overnight as low level moisturegradually increases over the region. Have made minor adjustmentsto the near term portion of the forecast for temperatures anddew points, otherwise little in the way of changes to thisupdate.Prev Disc...Stagnant boundary layer this evening and overnight will lead toslow moving showers across western and northern NH. These haveso far formed off terrain lift, forming a neat line along the VTGreen Mtns and now the Whites in NH. Can`t rule out a rumble ofthunder, but the lack of shear and more potent CAPE should keepthese mostly lightning free.Elevated instability remains overnight, and any energy crossingthe region tonight could pop up some isolated showers aftermidnight and daybreak. This seems most likely for portions ofsouthern and central NH. Between cloud breaks overnight, couldsee fog formation in the valleys once again, especially whererain fell this afternoon.
-- End Changed Discussion --&&.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH THURSDAY/...Marginal threat for localized flash flooding Wednesday, but itwill be highly dependent on training showers/storms.Parameters continue to show decent precip efficiency and weakstorm motion Wed afternoon. Surface instability builds latemorning from the mountains across the interior. HREF mean CAPEdevelops 700-1000 j/kg here, with the profile long and skinny.Some patches support up to 1500 j/kg. RH through the column hasimproved through the week, but low levels do remain borderlineat around 70 percent. PWATs have slowly been increasing earlythis week, and should push above 1.1 inch overnight, the current75th percentile. Warm cloud layer continues to be an itemholding back precip efficiency, remaining at or below 10kft.This can prevent warm cloud processes from really boostingsoaking rain.Looking at storm motion, cloud layer winds are slow, but overallshear should carry storms towards the coast. Looking at hiresmodel reflectivity, storm mode appears single cell to multicell,likely pulsing up then falling apart due to the lack of ventingability. But, backbuilding or training does look possible asshowers/storms refire behind those that tail off to the SE.For areas that overcome broader lift, lack of deeper clouds, anddry antecedent conditions, there will be that chance forperiods of heavy rainfall that could lead to minor flashflooding. HREF probs of greater than a half inch and inch of QPFin 3 hours are above 80 percent for much of the Whites andfoothills of ME/NH. Thus the marginal ERO from WPC fits the billfor tomorrow.With CAPE outweighing shear tomorrow and small inverted Vsoundings, could also see some storms create gusty winds asthey collapse initially. But this is also expected to be limited.Showers will develop late morning and continue to spread incoverage into the afternoon and evening hours, tapering afterdark as surface instability weakens. Some showers may hang oninto the mid to late evening hours as elevated CAPE remains.&&.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...Overview: We return to wetter weather as the 500 mb patternshows a ridge over the Atlantic blocking an upper low over theGreat Lakes through the extended period. This will result indaily chances for rain showers as multiple waves rotate aroundthis low and over our region. Right off the bat we could seesome particularly heavy rain that may cause some localizedflooding.Impacts:* Slow moving showers containing heavy rain may cause some localized flooding on Thursday.Details: Vertically stacked low pressure approaches the regionThursday draping an occluded front just to the west of area.Convergence along this front looks to generate a healthy band ofshowers with decent upper level forcing entering into theregion as we end up downstream of a 75 kt jet. A very moistairmass will also be entering the region with PWAT values of1.50-1.75" modeled across much of the area. These values areabove the 90th percentile for this date and approaching thedaily max. All these ingredients combined spell a decent setupfor heavy rain and have rightfully warranted a Marginal Riskfrom the Weather Prediction Center, however there are some otheruncertainties in this set up that we will have to considerbefore we can pin down where the best chance for this to occuris. As detailed in the short term section, Wednesday poses athreat for localized flooding as well. Areas that get a soakingthen, will be more prone to flooding during these showers. Thisis an important factor because antecedent conditions in our areaare generally dry with rivers running below normal. Ensembleguidance is showing low level flow of only around 15-20 kts andwith very little instability, mean CAPE values only around 200-300 J/kg and really only in southwestern New Hampshire, thispoints more toward a flooding scenario caused by a slow movingarea of showers that, due to its efficient rain processes and aplenty moist airmass, will likely contain some heavy rain. Withall that being said GFS and EURO ensembles are spitting out24-hr rain totals between 0.75 and 1" with less than 25% chancesof going over an inch, which isn`t super impressive. As ofright now this looks like urban and poor drainage areas wouldsee the best chance of flooding, due to the aforementioned dryantecedent conditions and rivers running below normal. However,this is likely to change in some places following Wednesday`sevents.Vertically stacked low pressure will continue to send frontsthrough the region along with shortwaves rotating around theupper low. This will necessitate shower chances through theremainder of the week and into early next week, however noteveryday will be a wash and some will be drier than others.Temperatures look to remain pretty steady with highs in the 70sand lows in the 50s each day. Can`t speak much on exact detailsabout the rain day to day at this time range, but it is worthmentioning that return flow from the high pressure offshore willkeep an onshore component to the wind for a majority of thisperiod so PWAT values are looking to stay up around an inch. Itis also worth mentioning that the low level pattern continues toshow flow rarely exceeding 20 kts and as we get into theweekend and early next week shower activity will become moreconvective in nature as we see clearer skies and warmer daytimetemperatures. Isolated flooding will have to remain in the backsof our minds as we continue through this upcoming stretch ofwetter weather.&&.AVIATION /03Z WEDNESDAY THROUGH SUNDAY/...Short Term...VFR. Some uncertainty on fog/low stratus potentialalong ME/NH coast tonight through Wednesday. Confidence in thisisn`t enough to include in TAFs at this time. Valley fog maydevelop again across the interior after showers this afternoonand early Wed AM. Showers and TS possible Wed late morning intoafternoon. These will be focused for interior terminals, but canexpect coastal sites to also see showers moving their waymid/late afternoon.Long Term...A mixed bag of restrictions is expected on Thursdayas an area of rain crosses the region. This will likely haveembedded areas of heavier showers and may even have someembedded thunderstorms as well, but this is less likely andwould be more confined to southern New Hampshire terminals. VFRlooks to prevail through the rest of the week, but isolated toscattered showers and thunderstorms will be possible eachafternoon. Winds remain 5-10 kts with gusts 10-15 kts throughSunday.&&.MARINE...Short Term...Below SCA. Warm, moist flow over the cooler SSTwill create patches of vis reduction due to fog through at leastthe next 36 hours.Long Term...Conditions hazardous to small craft are not expectedthrough Sunday. Fog over the waters is likely Thursday alongwith rain showers as a system crosses the region. Showers andthunderstorms are possible each afternoon this week. Winds areprimarily onshore at 10-15 kts through Sunday as high pressureremains over the atlantic and low pressure remains inland.&&.GYX WATCHES/WARNINGS/ADVISORIES...ME...None.NH...None.MARINE...None.&&$$NEAR/SHORT TERM...CannonLONG TERM...Baron