301 FXUS61 KBOX 101920 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 320 PM EDT Wed Oct 10 2018 .SYNOPSIS... */ Near and short term forecast updates only... Continued warm and humid into tonight. 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. High pressure will build in for the weekend, bringing dry but rather cool fall like weather. Showers may return during Monday or Tuesday. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... 3 pm update... Scattered cloud decks will become overcast overnight and lower. With any breaks and given light winds there is the possibility of some patchy ground fog. As highs exceeded expectations today by a couple of degrees (Boston reaching a high of 86). Lows will drop back down into the mid 60s. A fly in the ointment however is a potential backdoor cold front which will dip potentially into NE Massachusetts. With N winds behind, lows in the region behind the front are forecast to fall into the upper 50s to low 60s. && .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/... Big Picture... Atlantic high pressure retreats south. Northern jet stream digs south with falling heights over the northern USA. Upper low associated with Hurricane Michael or its remnants moves north from the Gulf of Mexico, then gets caught in the sinking mid-latitude flow and its movement turns toward the northeast and passes well south of New England. One shortwave from the Pacific Northwest sweeps east and over our area Saturday. A second shortwave from Northern Canada digs south to the Great Lakes and Northeast USA early next week. Upper contour heights are similar through Sunday morning but then show differences. In particular the second of the shortwaves crosses our area at different times depending on the model. The GFS is about 12 hours faster than the ECMWF but similar to the GGEM. Confidence is high Friday, then diminishes to low by Monday and Tuesday. Contour heights are above normal through Friday, then fall to below normal over the weekend and rise a little early next week. We agree with the trend to colder temps behind the Thursday night front and lingering through the weekend. Guidance temps show a warming trend early next week, but not as much as the height increase suggests. We will go with the slower warming. Concerns... Thursday night-Friday... Cold front moves through the region from 00Z to 06Z. Michael or its remnants moves northeast from coastal North Carolina following the upper flow as noted above. This takes the center across 38.5N/70W, well south of Nantucket during Friday. The plume of high PW air remains over Srn New England through midnight and along the South Coast through the night. This suggests showers and embedded local downpours for part of the night inland and all night near the South Coast. A SW 40-50 knot low level jet is along the south coastal areas Thursday night, then shifts south and offshore overnight as the cold front moves through. This may bring wind gusts of 30-40 mph in southern areas until the cold front approaches, then winds will diminish. We will continue the potential for showers/thunder/local downpours, diminishing most areas overnight. Winds then shift from the northwest for Friday. Temperatures will cool overnight as northwest winds bring in lower dew points inland. Less certain of that along the coast where precipitation will continue to moisten the air. Min temps from around 50 northwest to the lower 60s over the Islands. The plume of moisture will linger along the South Coast Friday morning, but should shift offshore during the afternoon. Expect dry weather in most of the area, but morning showers southeast of a Plymouth-Providence line. Finally, the passage of Michael will bring a period of high surf to the exposed South Coast, especially the Islands and east- facing coast of Outer Cape Cod. Some of this will linger into Saturday. Weekend... Weak shortwave and cold pool move across our area Saturday. Cross sections show medium depth of moisture with this shortwave, so a period of clouds is likely. But the air below 850 mb is expected to be dry so no precip is forecast. Temps aloft are forecast at an 850 mb equivalent of 0C to -2C. Max temps are based on this as well as the expected morning temperatures, adding 5-7F to the latter. High pressure overhead and fair skies should allow radiational cooling, and with dew points in the mid 30s to low 40s we expect min temps in the mid 30s to mid 40s. This means potential for frost in some areas, including northern Mass and the low spots/bogs in the coastal plain. Fair weather Sunday with mixed layer temps supporting max temps in the mid 50s to low 60s. Monday-Tuesday... As noted above, timing of the next shortwave and its cold front are in question. But expect clouds and showers to arrive either later Sunday night through Monday, or later Monday through early Tuesday. We will go with a compromise on timing, with chance pops for showers each day. && .AVIATION /19Z WEDNESDAY 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. 21z update... Rest of today... SCT-BKN MVFR to low-VFR CIGs with SW winds. IFR-LIFR holds along the S/SE coast especially in and over the Cape and Islands. Tonight... Lean CIGs becoming BKN-OVC, lowering MVFR to IFR. Held back on VSBY issues and LIFR expect along the S/SE coast including the Cape and Islands where BR / FG issues are likely to continue. Light winds. One last concern is a NE wind shift over NE MA which will creep in overnight. 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... Trend lowering CIGs overnight while becoming more BKN-OVC. Hold it at MVFR. No VSBY issues. Lower issues will emerge as -RA/RA moves in towards later Thursday. Concurrent increasing SW winds, gusts upwards of 25 kts possible going into late Thursday into Thursday evening before shifting NW. KBDL Terminal... Lowering CIGs overnight down to MVFR. Will bring -RA/RA in during the morning hours dropping conditions further towards IFR. Decent S winds, bit breezy. Can't rule out TSRA across the terminal, TEMPO LIFR. Trends continue into the evening hours before a NW wind shift overnight. Outlook /Friday Night through Monday/...Moderate Confidence. Friday Night through Saturday: VFR. Breezy. Saturday Night: VFR. Sunday: VFR. Breezy. Sunday Night: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. Monday: Mainly MVFR, with local IFR possible. Breezy. 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. 3 pm update... GALE WATCH posted. Marine fog and low clouds will be an issue through tonight. Perhaps scouring out as SW winds increase, 15 to 25 kts with gusts up to 35 kts, 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 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. Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Sunday: Winds less than 25 kt. Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Monday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. 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. 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...WTB/Sipprell NEAR TERM...WTB/Sipprell SHORT TERM...Sipprell LONG TERM...WTB AVIATION...WTB/Sipprell MARINE...WTB/Sipprell