KBOX issues Area Forecast Discussion (AFD) valid 2019-12-09T06:20:00Z 359 FXUS61 KBOX 090620 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 120 AM EST Mon Dec 9 2019 .SYNOPSIS... High pressure offshore will start a flow of increasingly milder and more moist air overnight. Showers will develop around daybreak and will turn into a soaking rain Monday into Tuesday, along with strong southwest winds in southeast Massachusetts and on all coastal waters. A cold front moves through Tuesday night, ushering in much colder air that will likely change rain to snow and linger into Wednesday with accumulations possible. Cold and dry into Friday before temperatures begin to moderate Friday night. Next system brings a period of rain by the weekend, with temperatures trending above normal. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 120 AM Update: Few sprinkles have appeared on KOKX WSR-88D and were lifting NE through southern CT. Not sure how much of this activity will reach ground as it approaches southern Hartford County and possibly southern parts of Tolland and Windham Counties too. Given temperatures (and road surfaces) are below freezing, since southerly flow has not begun in earnest, it's possible that we see some light icing in spots through about 3 AM. Special Weather Statement issued to highlight this potential. Otherwise, southerly flow is underway which has helped temperatures begin to rise, especially near coast, which will be trend overnight. Warm advection has also led to development of low clouds near CT Valley, as moisture has become trapped beneath inversion. High res guidance indicates these clouds will become more widespread overnight across interior, before spreading toward coast around daybreak. Still looks like any showers will hold off until around sunrise, which is supported by HRRR and to some degree 00z NAM. By then, temperatures will have risen to above freezing, so we do not anticipate any threat of icing. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TUESDAY/... *** Periods of moderate to heavy rain *** 345 PM update... Monday ... Strong thermal and moisture advection ahead of approaching short wave with 925 mb winds increasing to 60-70 kt. This results in PWATs surging to about 1.25 inches by end of the day. This synoptic scale lift combined with anomalous moisture advection will yield periods of moderate to heavy rain. This strong low level WAA results in surface dew pts rising into the low 50s across eastern CT/RI and eastern MA. These high dew pts combined with plenty of SSW wind should yield appreciable snow melt and combined with moderate to heavy rain at times, expecting lots of standing water for the late day/evening commute. Thus hydroplaning could become a concern. These increasing dew pts streaming over the snowpack will also increase the probability of patchy dense fog. Strongest wind gusts up to 30 mph will be over southeast MA where dew pts climb into the low 50s and easy low level inversion. Inland with dew pts only the 40s should be enough of an inversion to keep winds 20 mph or less. As for thunder, not expecting much if any given lack of elevated instability/poor mid level lapse rates. Monday night ... *** Periods of Moderate to Heavy Rain Likely *** Periods of moderate to heavy rain continue thru the evening as robust mid level short wave moves across the area and accompanied by 60-70 kts of SSW flow at 925 mb and advects PWATs up to 1.4 inches across RI and eastern MA. This strong low level WAA results in surface dew pts continuing to surge into the low and mid 50s. Thus more snowmelt combined with periods of moderate to heavy rain during the event will continue to yields lots of standing water on area roadways. SW winds could gusts up to 40 mph over southeast MA including Cape Cod and the Islands. This strong low level WAA pattern will result in temps remaining in the 50s overnight! These increasing dew pts streaming over the snowpack will also keep the risk of patchy dense fog. Short wave moves east of the region second half of the night with weak short ridging building across the area. Thus a downward trend in precip is likely after midnight. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... 345 PM update ... Highlights... * Cloudy but mild Tues with another round of showers mainly late day * Rain early Tues nite changes to a period of accumulating snow (potentially plowable in interior and urban corridor) overnight into Wed. Increasing confidence for adverse impact to Wed AM commute. * Cold and dry Wed nite through Fri. Moderating temperature and moisture levels Friday night. * Next system for the weekend, with rainy conditions and above- normal temps. Details... Tuesday: In the wake of expected rains Monday night, weather pattern remains unsettled for Southern New England. Most of the region will be the warm sector and a cloudy one at that. Will keep chances for showers going through the first part of the day, but tried to focus highest PoPs into the Likely range more tied with the cold front, which approaches our western counties late Tuesday. Cold frontal precip should prove much less than the rains for Monday/Monday night, with additional QPF of a quarter inch or less. Highs mid/upper 50s to near 60. Tuesday night into Wednesday: Colder air rushes southeastward Tuesday night, and as at least sheared-out mid-level vort max ripples northeastward from the central Appalachians, will be looking at rain changing to wet snow on the cold side of the cold front. Did note more ECMWF Ensemble members indicating snow for Tuesday night into Wednesday than 24 hours ago, with ensemble probabilities for 3" or more of snow in the 24-hour period 18z Tue - 18z Wed now up to 50% across western MA/western Hartford County, the hills in central MA and Tolland County eastward into the northern Providence suburbs. Recent trends in the GFS have also trended snowier in this period. Global models show anomalously strong mid and upper level jets (~ 100 kts at 500 mb, 150 kts at 250 mb, with Southern New England positioned in the favorable equatorward entrance region of each jet), and there are hints in the guidance at some bands (axis of 850 mb -EPV, some though unfocused FGen @ 850) during the overnight hrs. What remains uncertain is the timing of the changeover, forecast liquid-equivalent precip and the northwestern extent of accumulating snows. Each of the above factors renders confidence in snow accumulations at low to moderate levels. Did note that most models have again increased QPF in this period, though there may be a sharp cutoff in precip N/W of the Mass Pike and especially north of northern MA/Route 2 corridor. WPC's Day-3 accumulation seems fairly reasonable as far as a first-cut at accumulations goes. In spite of stated uncertainties, do think confidence is increasing in potential for an adversely impacted Wednesday morning commute for at least a part of the region, especially across the central part of the interior (Hartford/Springfield, Worcester/Providence into the I- 95 corridor). This is roughly where axis of greatest liquid- equivalent QPF seems to lie, though some variability in models exists to be re-evaluated with forthcoming guidance. Pavement's also apt to be wet at onset and sharply colder temperatures filtering that could freeze this before or during the period of accumulating snow, the potential for slippery travel into the AM commute remains a possibility. Wednesday Night through Friday Night: 1040+ mb high becomes anchored across much of New England. Quite chilly and dry through Thursday night with 850 mb temps in the -10 to -15C range. Will show highs in the 20s to near freezing, with lows in the teens. A few single digit lows are possible across northern MA. Southerly warm advection pattern commences Friday into Friday night as high pressure shifts into the Canadian Maritimes. Temperatures and moisture levels should begin to recover. Will see increased clouds into the nighttime hrs Friday night, and while the GFS brings in warm frontal precip late, likely too dry to support PoPs higher than low Chance. Saturday into Sunday: Weekend remains unsettled with the next chance of significant precip by the weekend, though models continue to deviate on timing. As mentioned, the GFS brings rain in as soon as Friday night, while the ECMWF is more into late Saturday into Saturday night, and the Canadian GEM is still slower than the ECMWF. Could be a decent rain maker for most of southern New England, with another round of gusty southerly winds across the waters. A return to above-normal temps looks likely for the weekend with 850 mb temps rising into the mid/upper single digits Celsius. Will show highs in the upper 40s to mid 50s with clouds and rain likely accompanying the warmest temps, though potential for even warmer with cloud breaks. && .AVIATION /06Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...High confidence. 06Z TAF Update: MVFR CIGS across much of interior and around Cape Cod will slowly expand into RI and eastern MA through daybreak. Conditions then steadily lower to IFR/LIFR as showers arrive near KBDL around 12Z and reach eastern MA coast by 14Z. Lowest conditions expected across interior due to presence of deep snowpack. These low conditions persist through tonight before some improvement Tue. LLWS this morning inland due to light winds at surface and 2kft winds 19040kt. Later today and tonight focus of LLWS shifts into RI and eastern MA as southerly low level jet increases to 50-60kt at 2kft. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Tuesday Night through Friday/... Tuesday Night: MVFR/IFR conditions possible. Breezy. SHRA likely, SHSN likely, PL likely, FZRA likely. Wednesday: Mainly VFR, with areas IFR possible. Breezy. Chance SHSN. Wednesday Night through Thursday: VFR. Windy with local gusts up to 30 kt. Thursday Night through Friday: VFR. Breezy. && .MARINE... Short Term /through Tuesday/...High confidence. 1230 AM update: ** Gale Warning Continues late Monday/Monday Night ** Overnight ... 1030+ mb high pressure over Georges Bank continues moving seaward overnight. Gradual increase in SSW winds. Dry weather and good vsby thru midnight but then risk of showers toward daybreak. Monday ... SSW winds increase to gale force in afternoon along with building seas. Areas of dense fog develop possible in the afternoon along with milder conditions. Periods of moderate to heavy rain limit vsby. Monday night ... SSW gales continue with periods of moderate to heavy rain during the evening along with areas of dense fog possible. Vsby may improve after midnight as rain moves offshore and SSW winds become more SW. Outlook /Tuesday Night through Friday/... Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 10 ft. Rain showers likely, patchy fog. Wednesday: Winds less than 25 kt. Local rough seas. Rain showers likely, snow showers likely. Local visibility 1 to 3 nm. Wednesday Night: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Areas of rough seas. Thursday: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Local rough seas. Thursday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Friday: Winds less than 25 kt. Local rough seas. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Warning from 6 PM this evening to 9 AM EST Tuesday for ANZ231>235-237-250-251-254>256. Small Craft Advisory from 2 PM this afternoon to 9 AM EST Tuesday for ANZ230. Small Craft Advisory from noon today to 9 AM EST Tuesday for ANZ236. && $$ SYNOPSIS...Nocera/Loconto NEAR TERM...JWD SHORT TERM...Nocera LONG TERM...Loconto AVIATION...JWD MARINE...JWD