Scientific Forecaster Discussion

000 fxus61 kgyx 230903 afdgyx Area forecast discussion National Weather Service Gray ME 403 am EST Sat Feb 23 2019 Synopsis... high pressure will crest over the region early today before shifting offshore. A warm front will approach the region from the southwest tonight and lift north into southern New England on Sunday spreading snow that will change to a wintry mix and rain before ending Sunday evening. Low pressure developing on this front will strengthen rapidly as it moves into the Canadian Maritimes on Monday resulting in very strong westerly winds that will bring much colder air into the region. Seasonably cold and dry conditions will continue for the remainder of next week. && Near term /until 6 PM this evening/... after a chilly start this morning under high pressure, expect a mostly sunny sky and temperatures warming back up above freezing... into the upper 30s and low 40s for most areas. Winds will be very light and shifting southerly as high pressure moves east. && Short term /6 PM this evening through Sunday night/... low pressure developing over the Texas Panhandle this morning tracks northeast through the Great Lakes and into Canada by Sunday. Ahead of this, a broad warm advection regime will spread snowfall into northern New England beginning Sunday morning. Timing of this keeps shifting back a bit, so delayed the onset until around daybreak for most of the area, perhaps a bit earlier in southwest New Hampshire. The later start time of the precipitation also signals a bit later arrival of clouds and a bit lower temperatures tonight. As the trough axis approaches the East Coast, secondary low pressure is expected to form and track through the Gulf of Maine. This will help to lock the cold air in place along and north of the low track... at least at the ground level. Warm advection aloft will change snow to freezing rain during the day Sunday. Winter weather advisories have been posted for most of the area due to the combination of light to moderate snow and light freezing rain. Ice amounts are not high enough to warrant a major threat beyond travel impacts. Higher resolution guidance seems to be performing a bit better on surface temperatures for Sunday and relied heavily on this as well as a mental model of temperature trends during similar events. There does remain considerable uncertainty in temperatures for some of our most populous areas including Manchester and Portland. For Manchester, current forecast is for subfreezing air to remain in place through mid morning bringing at least a brief window of freezing rain. However, it would not be entirely surprising if cold air hung on into the afternoon despite forecast models insisting on a warm up. Thus the Winter Weather Advisory will stay in place through the day with early cancellation possible if things warm up as expected. At Portland there could be some warmer air leaking in from the coast right around the time that precipitation starts changing form. Current forecasts have things warming above freezing before the transition, thus it GOES from snow to rain with no freezing rain in between. However, this depends on the placement and whims of the coastal front which could stay offshore and allow for a period of freezing rain. This is not in the current forecast, so Portland is not in the Winter Weather Advisory, but there is still a chance that freezing rain could occur there. Further north it will likely stay cold enough at all levels for an all snow event for the mountains and foothills of western Maine. In this area 6 to 8 inches is expected and a Winter Storm Warning has been issued. As the trough axis/surface low move east Sunday evening, expect winds to shift to the west. This will allow better low level mixing and temperatures likely warm above freezing for much of the area as precipitation exits quickly eastward. However, cold advection in the westerly flow will bring temperatures back down below freezing by morning with winds becoming quite strong. Tightening pressure gradient associated with the deepening low to our east will cause a 50+ knots low level jet to form. With cold advection and a downslope component to the winds, the likelihood of the stronger gusts mixing down increases during the overnight hours Sunday night. Will likely need a Wind Advisory at least beginning during the overnight and extending into the long term period of the forecast. && Long term /Monday through Friday/... 00z operational and ensemble guidance in relatively agreement with active scenario on Monday. A surface area of low pressure undergoes rapid cyclogenesis over the Canadian Maritimes during the day, dropping down to 968 mb. Meanwhile, a strong 1040 mb anticyclones will build east from the northern plains. This extreme pressure gradient, combined with strong cold air advection over the region and winds aligning through a deep layer will bring high wind gusts to the region Monday. Many areas will gust in the 40 to 45 kt range and possibly higher. This may lead to some power outages across the region. Strong wind gusts will continue Monday night, before gradually diminishing slightly late at night and Tuesday. The westerly winds will continue to bring upslope snow showers to the higher terrain in the mountains and foothills. This will be during a period of well below normal temperatures. Wind chill values will lower to well below zero in many areas. Headlines will likely be required for a portion of our forecast area due to the low wind chills. Models remain in disagreement on Wednesday. A relatively dry and brisk westerly flow will likely continue into Wednesday evening. However, the 00z GFS continues to depict a wave of low pressure forming and crossing through central New England and into the Gulf of Maine Wednesday night with light snow possible. High pressure will continue to gradually move east Thursday, before cresting over New England on Friday. This will continue to allow for below normal temperatures for this time of the year through the end of the work week. && Aviation /09z Saturday through Wednesday/... short term...VFR conditions expected today. Will see increasing clouds and lowering ceilings overnight with snow beginning from west to east by Sunday morning. Snow transitions to freezing rain at mht, con, and possibly psm, pwm, and Aug. Winds shift to the west Sunday evening and become quite strong overnight. Gusts to 30 or 40 knots are possible. Long term...westerly winds on Monday will likely gust over 45 kts...and continue through the evening with gradual slackening of the winds late Monday night into Tuesday. VFR likely to continue Monday-Tuesday with shsn still possible in the mountains. Shsn possible all areas Wednesday with localized MVFR conditions. && Marine... short term...high pressure moves east today with winds becoming southerly and then southeasterly. Southeast winds could reach gale force over the outer waters on Sunday before shifting to the west Sunday night. With the storm force winds expected on Monday and in collaboration with neighboring offices we have decided to focus headline decisions on the storm watch rather than the brief period of preceding southeasterly gales. Long term...westerly winds on Monday will likely gust near 50 kts and continue into Monday night. Have raised flags for storm watches across the region in a strong gradient with cold air advection. Winds will continue to be gusty for the rest of the work week with gales possible Tuesday into Wednesday. && Gyx watches/warnings/advisories... ME...Winter Weather Advisory from 3 am to 7 PM EST Sunday for mez018>020. Winter Storm Warning from 7 am Sunday to midnight EST Sunday night for mez007>009-012>014. Winter Weather Advisory from 7 am Sunday to midnight EST Sunday night for mez021-022. New Hampshire...Winter Weather Advisory from 3 am to 7 PM EST Sunday for nhz005>013-015. Winter Storm Warning from 7 am Sunday to midnight EST Sunday night for nhz004. Winter Weather Advisory from 3 am Sunday to midnight EST Sunday night for nhz001>003. Marine...storm watch from Monday afternoon through late Monday night for anz153. Storm watch from Monday morning through late Monday night for anz150-152-154. Storm watch from Monday afternoon through late Monday night for anz151. &&

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