944 FXUS61 KBOX 110306 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1106 PM EDT Wed Oct 10 2018 .SYNOPSIS... Muggy weather continues overnight. Moderate to heavy rainfall along with the potential for flash flooding ahead of a sweeping cold front Thursday into Friday. Remnants of Michael sweep SE of Nantucket around Friday morning. Expect gusty winds throughout the Thursday into Friday period. A pattern change to much cooler weather is anticipated this weekend into the middle of next week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... 1040 PM Update... Fog bank has been pushing further inland over the last few hours as seen on surface obs as well as GOES-East Nighttime Microphysics composite product before high clouds starting to encroach across SW CT into SE NY state. Also noting lower clouds moving across SW Maine into E central NH with the slowly approaching backdoor cold front working SWwd. Winds have shifted to NE at KSFM with a temp drop from 65 degs at 00Z to 57 degs at 02Z, while remaining at 70 degs at KPSM and 68 at KDAW (both in SE NH). However, some question how much further S this front will slide based on latest high res model data. For now, do have the front pushing into Cape Ann by around 06Z, then sliding as far S as E of KORE-KBOS by around sunrise. Beginning to note some improving visibility across Long Island and S coastal CT as the higher clouds move across. Will monitor this trend for any improvement across our S coastal locations. In the meantime, have kept Dense Fog Advisory for Nantucket for now. Updated cloud cover to reflect current short range trends, as well as incorporated overnight trends into the remainder of the overnight forecast. Previous Discussion... Main issue tonight is for the development of fog near the south coast, some of which may become locally dense. Have gone ahead and issued a special weather statement to cover this potential. We did go with a Dense Fog Advisory overnight across Nantucket where confidence highest. This may have to be expanded a bit further northwest depending on trends overnight. Across the rest of the region, enough SW flow in the boundary layer should keep any fog rather patchy. Otherwise,the other issue will be a backdoor cold front that most high resolution guidance drops southwest into northeast MA overnight. This should allow low temps to drop into the 50s in some of these areas by daybreak...while the rest of the region has low temperatures in the 60s. Dry weather prevails overnight...but a few showers may develop toward daybreak mainly near and north of the MA turnpike. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY/... */ Highlights... - Tropical moisture associated Michael streaming N - This ahead of a sweeping cold front across New England - Moderate to heavy rain midday Thursday through Friday - Locally heavy at times, especially with any thunderstorms - Potentially in a short period, flash flooding possible - FLASH FLOOD WATCH in effect Thursday thru Friday - GALE WATCH on the southeast waters also Thursday thru Friday */ Discussion... Overview... An area of low pressure and trailing cold front associated with a sweeping mid-level H5 trof will lift tropical moisture in addition to the remnants of Michael N across New England beginning Thursday and continuing through Thursday night. Michael's remnants along with strong and gusty winds will sweep S of the region as the cold front pushes through Friday morning taking everything out to sea by late Friday into Friday evening. Flooding... Heavy rainfall certainly possible, excessive in nature, contributing to flash flooding. Synoptic setup similar to that of a predecessor rainfall event. Anomalous, tropical-origin precipitable waters that are poleward of Michael, roughly +4-5 standard deviations. H925-85 S moisture transport convergently focused along a low-level baroclinic zone, that being a stalled backdoor frontal boundary to the NE and a sweeping cold front from the W. Approaching mid-level H5 trof along with broader diffluence aloft and support via right-rear quadrant of an attendant upper level jet, ingredients supportive of deep layer ascent of a very moist column, on up through H3, with the warm layer extended up to around 13-14 kft undergoing tremendous lift. Coupled with negative showalters / lifted indices, and there's indications of updraft support. Add in terrain influences given S flow. Conceptual forecasting over model guidance spread. Also additional items of note such as recent wet weather conditions, low flash flood guidance (2-3" over a 1-6 hour span), and tides running a bit higher with the new moon (10a / 10p for Newport and New Bedford, while 3a / 3p for Nantucket). Coastal slower drainage possible with high tides with any excessive rainfall in addition to the flash flood threat. FLASH FLOOD WATCH posted for S New England broken down in segments that drop off with time towards the SE coast. Opted to take the maximum of all rainfall guidance blended with the consensus of WPC / RFC rainfall guidance. This outputs an average of 1 to 2 inches across the interior while upwards of 3 to 5 inches over SE coastal New England especially Nantucket and Marthas Vineyard. Locally higher amounts throughout with any thunderstorm activity and / or terrain influences, as well as low level convergent focus along frontal boundaries. Feels this goes well with maximum rainfall as advertised by HREF. In a short period of time and noting sensitivity of potential rain- fall as the area is saturated, flash flooding is certainly possible. Yet a change in Michael's track and / or perhaps a faster sweeping cold front could take rainfall amounts downward. Forecast can easily adjust, however given signals and potential threat, while difficult to nail down precisely, felt a FLASH FLOOD WATCH for the entire area was warranted. Winds... Gale force winds, both isallobaric and pressure gradient driven in response to Michael lifting NE along a sweeping cold front, mainly over SE New England and adjacent waters. Gusts up around 45 mph (40 kts) possible over two periods: 1.) SW late Thursday into Thursday evening, especially given precipitation-drag and threat of thunder- storms, and 2.) NW around Friday morning with undercutting colder air and steepening boundary layer lapse rates, Michael just passing SE of Nantucket. Can't rule out some isolated wind damage with a higher risk over SE New England especially during and after the passage of Michael SE of Nantucket, the enhancement of the NW wind profile driving behind the storm briefly as it exits out into the Atlantic. Marine... Michael passing SE of Nantucket around Friday morning. Likelihood of gales with potential gusts up to 40 kts, initially SW late Thursday into Thursday evening, a lull overnight, then NW around Friday morn with the passage of Michael. Wave heights at their height of 7 to 12 feet for the S / SE waters around Friday evening after passage of Michael as the NW winds drive, diminishing as we go towards Saturday morning. GALE WATCH posted for the SE waters for the two periods, noting that there will be a lull in between around Thursday night prior to Friday morning. GALE WATCH Thursday through late Friday. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights... * Pattern change to much cooler temperatures begins this weekend * Mainly dry this weekend other than a few brief showers Sat * A period of showers possible sometime Mon and/or Tue Details... The upper level ridge off the mid Atlantic coast that has brought unseasonably mild temperatures for quite sometime will get squashed this weekend into the middle of next week. This a result of several pieces of strong shortwave energy...helping to carve out northeast troughs. The result will be a pattern change to much cooler weather this weekend into the middle of next week. Temperatures will average at or below normal during this time. Mainly dry weather this weekend...but a shortwave may result in a few showers Saturday. Highs this weekend will only be in the 50s. Low temperatures will be in the 30s to the lower 40s and some frost headlines are possible across the normally coolest outlying locations. A cold front will likely bring a period of showers early next week. Specific timing is uncertain but models show the potential sometime Mon into Tue. && .AVIATION /03Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/...Moderate confidence. Overnight... Mainly LIFR-VLIFR CIGS and MVFR-LIFR VSBYS along S coastal MA/RI with areas of dense fog across Cape Cod/KMVY and KACK. MVFR-IFR CIGS as far N as KOWD-KORH-KBAF and into the northern portion of the CT Valley. Otherwise, may see local MVFR CIGS across the remainder of the region (NE and N central MA). Will continue to monitor for a period of fog moving SW into NE Mass after 06Z behind a backdoor cold front. Thursday through Thursday night... Lowering conditions with onset of -RA/RA, localized +RA. Overall MVFR-IFR with TEMPO IFR-LIFR with RA/+RA. Possible TSRA. Will see SW winds on the increase especially over the S/SE coast with potential gusts 30-35 kts, can't rule out 40 kt gusts with RA/TSRA. RA/TSRA activity will be heaviest and prolonged over SE New England. Friday... Lull in winds into the morning, shifting NW and increasing with passage of Michael. Especially over the SE coast, gusts 30-35 kts with potential higher gusts up to 40 kts. W to E improvement of MVFR-IFR towards MVFR to low-end VFR. RA/TSRA activity concluding over SE New England early on. KBOS Terminal...Moderate confidence in TAF. We will have to watch for NE winds and IFR to even LIFR conditions for the morning push behind a backdoor cold front. KBDL Terminal...Moderate confidence in TAF. Outlook /Friday Night through Monday/...Moderate To High Confidence. Friday Night: VFR. Breezy. Saturday: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA. Saturday Night: VFR. Sunday: VFR. Breezy. Sunday Night: Mainly VFR, with areas MVFR possible. Slight chance SHRA. Monday: Mainly MVFR, with areas VFR possible. Chance SHRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/...High confidence. 1040 PM Update... GALE WATCH remains in effect for all waters. Marine fog bank and low clouds will be an issue through overnight, though may improve after midnight as SW winds increase. Expect winds from 15-25 kt with gusts up to 35 kt, locally stronger up to 40 kts with moderate to heavy rain, potential thunderstorm activity, ahead of a sweeping cold front late Thursday into Thursday night. As it progresses out to sea into Friday morning, the remnants of Michael will sweep quickly NE just SE of Nantucket. Winds shifting and gusting out of the NW, there is a second period possible of 20-30 kt winds with gusts as high as 40 kts. Will see winds diminish going into Friday evening. Highest seas expected late Friday behind Michael. Wave heights as high as 7 to 12 feet on the outer waters. Outlook /Friday Night through Monday/...Moderate to High Confidence. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers. Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Sunday: Winds less than 25 kt. Sunday Night: Low risk for Small Craft Advisory winds with local gusts up to 30 kt. Seas locally approaching 5 ft. Slight chance of rain showers. Monday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of seas approaching 5 ft. Chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Flash Flood Watch from Thursday morning through late Thursday night for CTZ002>004. MA...Flash Flood Watch from Thursday afternoon through Friday morning for MAZ017>021. Flash Flood Watch from Thursday morning through late Thursday night for MAZ002>016-026. Dense Fog Advisory until 8 AM EDT Thursday for MAZ024. Flash Flood Watch from Thursday afternoon through Friday afternoon for MAZ022>024. RI...Flash Flood Watch from Thursday afternoon through Friday morning for RIZ001>008. MARINE...Gale Watch from Thursday afternoon through Friday evening for ANZ231>235-237-254>256. && $$ SYNOPSIS...Frank/Sipprell NEAR TERM...Frank/EVT SHORT TERM...Sipprell LONG TERM...Frank AVIATION...Frank/Sipprell/EVT MARINE...Frank/Sipprell/EVT