Scientific Forecaster Discussion

000 fxus61 kaly 240914 afdaly Area forecast discussion National Weather Service Albany New York 414 am EST Thu Jan 24 2019 Synopsis... mild temperatures and rain are expected today. The rain will be moderate to briefly heavy and could result in some flooding especially south and east of the capital region. Colder air will cause the rain to turn to snow this afternoon and evening before it ends. It will turn much cooler Friday into Saturday with some mainly light snow showers possible through the weekend. && Near term /through tonight/... ..Flash Flood Watch in effect through 6 PM for western New England, the Taconics, and the Mid-Hudson valley... As of 400 am, steady rainfall continues over southwest flow upslope-favored locations such as the southern Adirondacks, southern greens, and northern Berkshires, as well as the Lake George Saratoga region. Mesonet obs show that anywhere from 0.75 to 2.00 inches of rainfall has occurred in these areas already. This is a fairly impressive system from a moisture standpoint with pwats forecast to reach 1-1.25 inches over much of the forecast area today, which is +3 to +4 South Dakota per the naefs. Equally impressive are the 850 mb v-anomalies which are forecast to reach +2 to +3 South Dakota. 850 mb wind speeds may be as high as 85 kt per the NAM over southern parts of the forecast area. Where rainfall is spottier early this morning, it will fill in from west to east as an elongated area of strong 850 to 700 mb frontogenesis spreads into the region, along with an increasingly favorably positioned right entrance region of a strong upper level jet. This will allow a period of moderate rainfall to occur throughout the forecast area during the morning into the early afternoon. Cams such as the latest couple runs of the 3km NAM and the hrrr are indicating a narrow cold frontal rain band crossing the region from west to east roughly 15-19z. Enhanced rainfall rates can be expected if this band materializes, but the prospects for enhanced wind appear low given the strong inversion and lack of instability. Expecting a basin average additional 0.50-1.50 inches of rain today, bringing storm totals to 1-2 inches. Amounts up to 3 inches are possible in the favored upslope areas of the southern Adirondacks and southern greens. See the Hydro section below for discussion on flood potential. With strong southerly flow, high temperatures will be boosted into the 40s to low 50s in many areas, except staying around 40 or below in the southern Adirondacks. Colder air begins to spread in from west to east in the afternoon hours behind the frontal boundary, which will change rain to snow before ending. The deeper moisture and better forcing for ascent will depart before temperature profiles are cold enough to support snow, but minimal accumulation is possible, generally around an inch or less, possibly a couple of inches above 1500 feet elevation. Westerly winds will become strong and gusty behind the front this afternoon and evening. Any lingering snow showers should diminish after 00z, except for some lingering activity over the high terrain of the Adirondacks/Greens. Lake effect parameters become marginally favorable late tonight with southwesterly flow, and a surface trough may focus a band over northern Herkimer County, with additional light accumulation possible. Cold advection will send low temperatures back down into the mid teens to mid-20s, which could result in wet spots freezing back up again. && Short term /Friday through Saturday night/... anomalously strong 500 mb closed upper low will stretch from Hudson Bay to southern Ontario during the short term, while broad troughing will dominate the flow pattern east of The Rockies. A shortwave trough will rotate around the base of the upper low Friday, with a weak surface low/trough parallel and just north of the St. Lawrence Valley. This trough will likely focus a band of lake effect snow over northern Herkimer and Hamilton counties into Friday night and early Saturday as moderate lake-induced instability develops. The band may oscillate during this time period, possibly shifting back south Friday night and back north again Saturday morning, which should limit accumulations in any given spot, but winter weather advisories may be needed in future forecast cycles. Elsewhere, deep mixing is forecast as h850 cold advection occurs coincident with peak diurnal heating. This could result in some convective snow showers or squalls spreading downstream of the lake effect areas into the valleys by the afternoon with potential highlighted by positive values of the snow squall parameter. A limiting factor is the lack of a sharp surface boundary upon which to focus the snow showers. It will be breezy as well. Seasonably chilly Friday night, although partial cloud coverage and a respectable pressure gradient should keep temperatures from falling too much. Lake effect activity may be ongoing Saturday morning before shifting north as winds back toward southerly by the afternoon. This will be the result of a strong pv anomaly shifting southward across Ontario toward Lake Superior into Saturday night. Temperatures won't be impacted to much during the day Saturday as a cold airmass remains in place with below-normal highs in the mid-teens to mid-20s. The southerly flow will increase Saturday night along with cloud cover, so low temperatures won't fall off too much from saturday's highs. Some upslope snow showers will become possible over the southern Adirondacks late. && Long term /Sunday through Wednesday/... large upper level trough will dominate across the Great Lakes and into the northeast during the long term period. As a result, shortwave troughs rotating around the main upper level low over Canada will swing across the region from time to time, allowing for some snow showers and helping to enhance lake effect precipitation. At the start of the long term, upper level energy will be moving across Ontario and Quebec and a surface cold front will move across the region for Sunday into Sunday night. This will allow for some scattered snow showers, with the best chance across northwestern parts of the region, where some lake moisture will aid in the development of snow showers. Temps look fairly seasonable for Sunday, although much colder temps will arrive behind the front for Monday, when temps will only reach into the teens and 20s. Most of Monday looks fairly quiet, although the break will be brief, as the next system will be quickly heading towards the area from the Great Lakes for Tuesday into Tuesday night. There are some differences within the model and ensemble guidance on just how far south this next system digs. This will determine if temps warm up for Tuesday or if they remain fairly cold. Some models suggest there could be a steadier period of snow, others suggest just passing snow showers (and possible even rain showers for southern areas). For now, will play the forecast in the middle until models start to come into better focus. Behind this system, very cold weather is expected for the middle to latter part of the week. Although it will be dry for most areas, some bands of lake effect snow will develop off the eastern Great Lakes. Considering the very cold temperatures and persistent flow in place, some significant accumulation is possible for the western Adirondacks. && Aviation /07z Thursday through Monday/... approaching storm system will continue to bring periods of rain through the remainder of the overnight hours. Although flying conditions have generally been VFR/high end MVFR, they will become solidly MVFR and eventually IFR for all sites by daybreak for both visibility and ceilings. Surface winds will be variable, with light or calm conditions at kgfl/kpou/kpsf, while kalb will see southerly winds around 10-15 kts. At 2 kft, southerly winds will be 40-45 kts, so will include low level wind shear for all sites. A period of moderate to heavy rainfall is expected for Thursday morning for all sites. The lowest visibility is expected to be at kpsf/kpou, although all sites look IFR through the morning hours. Surface winds will generally be around 10 kts, although some higher winds are possible at kalb, with low level wind shear continuing in place. The heaviest rain will start to taper off after midday, but light rain will continue into the afternoon with MVFR conditions. As temps fall, precip could even changeover to snow at kpsf, although it appears that temps won't cool down fast enough for this to occur at the valley sites. Winds will switch to the west for the afternoon and be around 10-15 kts with some higher gusts. Once precip tapers off by late afternoon or early evening, flying conditions should return back to VFR for all sites. Skies will start to clear out for Thursday night and westerly winds will continue to be gusty for all sites. Outlook... Friday: moderate operational impact. Chance of shsn.Friday night: no operational impact. No sig weather. Saturday: no operational impact. No sig weather. Saturday night: low operational impact. Slight chance of shsn. Sunday: low operational impact. Slight chance of shsn. Sunday night: moderate operational impact. Chance of shsn. Monday: low operational impact. Slight chance of shsn. Monday night: moderate operational impact. Chance of snow. Tuesday: high operational impact. Likely snow. && Hydrology... Flash Flood Watch has been expanded to include southern Vermont and the northern Taconics while remaining in effect for western Massachusetts, northwest Connecticut, the Mid Hudson valley, and the central/southern Taconics this morning through this evening. Moderate rain will continue today, becoming steadier in areas where it has been light overnight, as it becomes more focused on a low- level front. An additional 0.50 to 1.50 inches of rainfall is expected today, with storm total amounts generally from 1-2 inches. Higher amounts around 3 inches are possible in the southern Adirondacks and southern Green Mountains. The rain could be briefly heavy along a narrow cold frontal rain band from the late morning into the mid-afternoon. The rainfall will diminish from west to east this afternoon into the evening, changing to snow before ending. This rainfall, along with the melting snow, will allow for some rises on rivers and streams. Much of the precipitation will likely be absorbed by the snowpack, especially over the southern Adirondacks and Lake George Saratoga region where temperatures will only rise into the upper 30s to low 40s. However, south and east of Albany (where snow pack is more limited) more of the precipitation will be converted to runoff. With a frozen ground in place, some minor flooding is possible, including along the Housatonic River. Some of the snowpack over the northern Taconics and southern greens could begin to melt as dewpoints reach the mid- 40s this morning, so minor flooding will also be possible along the Walloomsac and the hoosic. In addition, urban/small stream/poor drainage flooding is possible across the area, especially since drains are covered by snow banks in many areas. Some isolated flash flooding also cannot be ruled out. With river rises and ice movement on some rivers, ice jams can not be ruled out. However the limited amounts of total runoff should prevent this from being a major concern and this will be more of an isolated issue. Behind this storm system, colder weather will return to the region for Thursday night into the weekend. Any additional precip will be in the form of snow and fairly light. For details on specific area rivers and lakes, including observed and forecast river stages and lake elevations, please visit the advanced hydrologic prediction service /ahps/ graphs on our website. && Aly watches/warnings/advisories... CT...Flash Flood Watch from 6 am EST this morning through this afternoon for ctz001-013. New York...Flash Flood Watch from 6 am EST this morning through this afternoon for nyz060-061-064>066. Massachusetts...Flash Flood Watch from 6 am EST this morning through this afternoon for maz001-025. Vermont...none. && $$ Synopsis...Thompson near term...Thompson

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