Area Forecast Discussion

WHAT HAS CHANGED

Increased PoPs a bit at points tonight into Monday morning across areas south of Lake Ontario.

KEY MESSAGES

1) Significantly colder weather will continue through Monday night, with unsettled conditions lasting into Monday morning.

2) Welcomed period of dry weather beginning Monday night.

DISCUSSION

KEY MESSAGE 1...Significantly colder weather will continue through Monday night, with unsettled conditions lasting into Monday morning.

A deep longwave trough will continue to dig across the Great Lakes tonight and New England Monday...resulting in 850 mb temperatures bottoming out near -12C Monday morning...before starting to slowly modify Monday afternoon as low level ridging builds into the area. This will result in well below normal highs in the upper 30s and lower 40s today...with readings then struggling to get out of the mid to upper 30s in most areas on Monday. Meanwhile lows both tonight and Monday night will be in the 20s...with even some teens possible across the interior of the Southern Tier and North Country Monday night.

In terms of pcpn chances through Monday...these will come primarily in three different batches. The first will be this afternoon into very early this evening...as diurnal heating of our chilly airmass and the passage of a shortwave will lead to continued scattered to numerous rain showers...with some wet snow mixed in at times. The greatest concentration of showers will lie along an expected low level convergence zone extending from the Niagara Frontier east- southeastward across the Finger Lakes...where some rumbles of thunder and small hail will also be possible given steep low level lapse rates and weak instability.

The second batch will then come this evening...when a secondary cold front and another shortwave will slip southeastward across the area...with these generating another round of scattered to numerous showers that will change to all snow over time with the loss of diurnal influences and continued cooling of our airmass. The greatest coverage of these will be expected south of Lake Ontario... with local maxima possible immediately south/southeast of both lakes due to some additional lake enhancement/upsloping. The third will come later tonight and early Monday in the form of some scattered upslope and lake effect snow showers across the higher terrain and southeast of the lakes. This last batch will then fade out from northwest to southeast during the balance of Monday as high pressure and drier air builds across the area...with a dry night then following for Monday night.

In terms of snowfall accumulations...strong mid/late April diurnal influences and the warm ground conditions (soil temps running in the upper 40s to lower 50s per NYS Mesonet obs) will largely help prevent these this afternoon. The loss of daytime heating and continued cooling of our airmass should then allow for some minor accums of up to an inch on elevated/grassy surfaces across the higher terrain tonight and very early Monday...with renewed diurnal effects and the diminishing trend to the pcpn then preventing much (if any) in the way of additional accumulation after about mid Monday morning.

KEY MESSAGE 2...Welcomed period of dry weather beginning Monday night.

A shortwave ridge will pass across our region Monday night, to be followed by a dampening shortwave trough...that may bring scattered showers to our region Tuesday night. Behind these two features a more broad area of high pressure, both at the surface and aloft, will build across our region through at least Friday morning...if not longer. This broad area of high pressure will bring fair weather with day to day warming.

While the majority of the ensemble members of the GEFS/ENS/GEPS do maintain the ridge into the start of next weekend (with any precipitation along frontal boundaries), there is a growing number of ensemble members, especially among the GEFS, that develop and strengthen a cut off low over the Northeast/eastern Canada. If this scenario were to occur, cooler and unsettled weather would be likely to persist for several days across our region.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Diurnal heating of our chilly airmass and the passage of a shortwave disturbance will lead to continued scattered to numerous rain showers this afternoon...with some wet snow mixed in at times. The greatest concentration of showers will lie along an expected low level convergence zone extending from the Niagara Frontier east- southeastward across the Finger Lakes...where some rumbles of thunder and small hail will be possible given steep low level lapse rates and weak instability. Expect a mix of VFR to MVFR conditions to prevail this afternoon...with the lower conditions generally found within any showers.

A secondary cold front and another shortwave will then slip southeastward across the area this evening...with these generating another round of scattered to numerous showers that will change to all snow over time...with the greatest coverage of these expected south of Lake Ontario. Flight conditions will lower to MVFR and possibly even some IFR within these showers. Later tonight and early Monday some scattered upslope and lake effect snow showers will then follow across the higher terrain and southeast of the lakes...with additional localized reductions to MVFR/IFR possible within these. Outside of any showers...a general mix of MVFR/VFR will prevail.

On Monday high pressure and drier air will build across the area. This will result in any leftover snow showers diminishing from northwest to southeast...with flight conditions improving to VFR in a similar manner.

Outlook...

Monday night...VFR.

Tuesday...Mainly VFR, with a chance of rain showers mainly late Tuesday and Tuesday night.

Wednesday and Thursday...Mainly VFR.

Friday...Mainly VFR with a low chance of afternoon showers.

MARINE

Elevated (15-20 knot) west to west-northwesterly winds across the region this afternoon will veer more northwesterly tonight following the passage of a secondary cold front. On Monday winds will tend to diminish some in the morning, then will back to westerly during the afternoon...when another brief round of 15-20 knot winds will be possible across eastern portions of Lake Ontario. Lighter winds will then follow for Monday night as the axis of surface high pressure crosses the Lower Great Lakes.

Given the above...Small Craft Advisories are in effect as outlined below.

BUF WATCHES / WARNINGS / ADVISORIES

NY...None. MARINE...Small Craft Advisory until 11 PM EDT this evening for LEZ040- 041. Small Craft Advisory from 11 PM this evening to 2 PM EDT Monday for LOZ043. Small Craft Advisory until 2 PM EDT Monday for LOZ044-045.