Area Forecast Discussion
Issued by NWS Albany, NY

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000 FXUS61 KALY 190239 AFDALY Area Forecast Discussion National Weather Service Albany NY 1039 PM EDT Sat Aug 18 2018 .SYNOPSIS... A cold front will move southward through the southern New York and southern New England this evening. A drier airmass will move in behind the front this evening. Sunday and Monday are expected to be mainly dry and much less humid, although a few showers are possible from time to time over far southern areas. && .NEAR TERM /THROUGH SUNDAY/... As of 1030 PM EDT, GOES-16 advanced microphysics imagery shows a decent amount of mid level clouds across the forecast area this evening with some higher level clouds streaming across southern New York. These clouds, along with breezy northerly winds, should inhibit any fog formation overnight. Temps are currently in the upper 50s to mid 60s, almost 10 degrees cooler than this time last night. Prev Disc...As of 745 PM EDT, cooler and drier air continues to filter into the region from the north, resulting in lower dew points and breezy conditions. Clouds continue to linger, however, with some low to mid level moisture evident in the GOES-16 layered WV imagery. Temperatures are currently in the 60s but are expected to dip into the upper 50s to mid 60s overnight. This is on track with the current forecast so have freshened up the hourly temps and sent the update to NDFD and web servers. Further details regarding the forecast follow... As of 145 PM EDT, showers and thunderstorms are ongoing ahead of a cold front from the Berkshires southwestward to northeast Pennsylvania. The cold front will continue to push southeast through the remainder of the forecast area later this afternoon and this evening. The front will be intersecting a still-moist airmass characterized by PWATs of 1.75 to 2.00 inches, so it will be accompanied by scattered showers and thunderstorms. Some of the storms still have the potential to produce locally heavy rainfall, especially along and south of a line from the southern Berkshires to western Ulster County where more favorable frontal timing may allow for some respectable surface-based instability of 1000-2500 J/kg to develop given dewpoints in the low 70s and temperatures heating up into the upper 70s/low 80s. The front should be fairly progressive which will limit a more substantial flash flooding threat, but will have to especially watch areas that saw heavy rain on Friday for localized flooding concerns. Deep-layer shear values forecast to be 25 to 30 kts presents a marginal opportunity for storms to be organized, with several CAMs showing a broken line of convection occurring along the front. Isolated gusty winds cannot be ruled out. Main threat period for gusty winds and heavy rain appears to be roughly through 22Z. For the areas north of the frontal boundary, a drying trend is expected with breaks of sunshine towards evening in the wake of the front. High temps are a little tricky, but have sided away from the MET MOS guidance which appears to be a cool outlier. Highs are expected to range from the upper 60s to lower 80s. Tonight, we`ll be in a low-level northeasterly flow regime in the wake of the front as the deeper moisture shifts south of the region. A couple of lingering showers are possible across the extreme southeast portion of the forecast area. To the north, it is a tricky call with respect to sky cover as some guidance indicates a midlevel cloud deck developing in the cold advection. Outside of this area, some patches of fog are possible across northern zones as high pressure builds from the north. It will be noticeably less humid with dewpoints in the mid 50s to mid 60s with lows in the mid 50s to mid 60s. The frontal boundary is expected to stall somewhere in the vicinity of the Delmarva Sunday, while a weak mid-level trough exists over the mid-Atlantic into the Northeast. Will retain low chance PoPs over southern portions of Ulster/Dutchess/Litchfield Sunday to account for possible further shifts northward of a wave of low pressure along the frontal boundary. For the remainder of the forecast area, dry east to northeast flow at low/mid levels should keep things dry, although we may see some high clouds. Highs Sunday mainly in the upper 60s to upper 70s. && .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/... Sunday night, departing upper low near the I84 corridor will exit off the New England coast as ridge of high pressure at the surface will likely build in from the north-northeast. This may bring some additional moisture off the Atlantic into our New England counties, other the remainder of the region this high should allow for good radiational cooling conditions under partly cloudy to mostly clear skies. Have blended with guidance and previous forecast with near seasonal normals. Dewpoints settle back into the 50s. Higher confidence in a dry forecast and more clearing skies for the entire forecast area Monday into Monday night. Both the ridge at the surface and aloft increase so tranquil and near seasonable temperatures for the later half of August. Tuesday, global models are a little slower with the approach of the next upstream storm system set to impact the central and northern Mississippi River Valley. So we will reduce the onset of PoPs/Wx in the morning hours and continue with PoPs into the afternoon hours with the higher values to the west of the Hudson River. Clouds will increase through the day as temperatures should climb toward 80F for valley locations and low-mid 70s elsewhere. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... An impressive low pressure for late August moves up the Saint Lawrence River Valley Tuesday night into Wednesday. Initially, precipitation associated with the system`s warm front may struggle to reach into eastern NY/western New England as high pressure maintains control over eastern New England. However, by 06z Wednesday, enough warm air advection associated with the system`s 35- 45kts 850mb jet should push northeastward into our region leading to widespread precipitation. We should be within in the warm sector during the overnight hours and with dewpoints very mild in the mid - upper 60s, it should be a humid night. PWATS look to be over 1.5 - 2 inches with the Hudson Valley potentially over 2". It`s worth noting that the GFS and Canadian suggest a stronger 500mb height and wind field associated with this low Tuesday night into Wednesday over the EURO. These models suggest that the initial neutrally titled 500mb trough strengths, forms a closed low over southern Canada and turns negatively titled heading into 12z Wednesday as the trough axis moves through eastern NY/western New England. Should this occur, we may have some strong to even severe weather to contend with Wednesday morning. While instability is lacking with under 1000J/kg of MLCAPE showing on most pieces of guidance, this could be a high shear low CAPE environment as 0-6km shear looks to be 40-45kts thanks to the impressive 500mb wind field of 50-55kts. Mid-level lapse rates are decent as well ranging 5.5- 6C/km. Therefore, the period from 12z-18z Wednesday will need to be monitored for a potential line of strong to severe storms as the trough axis/surface cold front progresses through eastern NY/western New England. Given the high humidity, any storm will be capable of heavy downpours. By Wednesday afternoon and night, it looks to turn breezy as the impressive surface low (which its 990-995mb MSLP 3-4 standard deviations below normal for this time of year according to the GFES) moves through northern New England/southern Canada leading to a decent pressure rise/fall couplet and isallobaric wind field. Increased wind gusts to 20-30mph for Wednesday afternoon to reflect this threat, especially in the Dacks and southern VT where the isallobaric winds looks to be strongest. Cold advection will lead to gradually falling temperatures Wednesday afternoon as well and we should see a drastic drop in humidity heading into Wednesday night. Overnight lows should be comfortable falling into the 50s. Pleasant conditions remain in place Thursday and Friday as large scale high pressure and weak ridging moves in from the Midwest. Temperatures should be closer to normal for late August only reaching into the upper 70s both days. Humidity level remain low as well thanks to the northerly flow. The weak ridge axis should move into eastern New England heading into Saturday which should lead to to west-southwesterly flow aloft. This in turn should allow temperatures to warm up a few degrees Saturday reaching into the low 80s. For now left a dry forecast but a few shortwave embedded in the overall flow aloft will need to be watched. && .AVIATION /03Z SUNDAY THROUGH THURSDAY/... Drier and cooler air, along with breezy conditions, will continue to filter into the region from the north overnight in the wake of a cold front. These breezy conditions, along with lingering low to mid level clouds/moisture should inhibit fog formation. Outside of MVFR/IFR CIGS at KPSF/KPOU overnight, conditions will be VFR through the forecast period. All sites should improve to VFR after 12Z/Sun. Outlook... Sunday: Low Operational Impact. Slight Chance of SHRA. Sunday Night: No Operational Impact. NO SIG WX. Monday: No Operational Impact. NO SIG WX. Monday Night: No Operational Impact. NO SIG WX. Tuesday: Moderate Operational Impact. Chance of SHRA...TSRA. Tuesday Night: High Operational Impact. Definite SHRA...TSRA. Wednesday: Moderate Operational Impact. Chance of SHRA...TSRA. Wednesday Night: No Operational Impact. NO SIG WX. Thursday: No Operational Impact. NO SIG WX. && .FIRE WEATHER... After a wet day Friday and southern portions on Saturday, a drying trend will occur from north to south in the evening, setting the stage for a cooler and mainly dry period Sunday and should last into Tuesday morning. High temperatures will be near to slightly below normal Sunday and Monday with a light east to northeast wind. RH values will bottom out mainly in the 50 percentile. && .HYDROLOGY... Much of the region saw rainfall on Friday and southern areas during Saturday, with several spots receiving in excess of an inch and some areas across Columbia and Berkshire Counties seeing two to five inches of rainfall. Showers and thunderstorms will exit and weaken our southern zones by early evening. Waters will continue to recede into the remainder of the weekend and into Tuesday morning. 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...None. NY...None. MA...None. VT...None. && $$ SYNOPSIS...11/Thompson NEAR TERM...11/Thompson/JVM SHORT TERM...BGM LONG TERM...Speciale AVIATION...JVM FIRE WEATHER...BGM/Thompson HYDROLOGY...BGM/Thompson is the U.S. government's official web portal to all federal, state and local government web resources and services.