Le Roy, NY 14482 Weather

What Has Changed

No major changes with this forecast update.

Key Messages

1) Dry and tranquil weather will prevail through this weekend.

Discussion

KEY MESSAGE 1...Dry and tranquil weather will prevail through this weekend.

High pressure at the surface and aloft exits generally E'ward as a shortwave trough aloft continues to move SE'ward and approaches from southwestern QC, Lake Huron, and vicinity during the predawn hours of this morning. The only consequence is still expected to be the development of isolated to scattered mid/upper-level cloud cover amidst moist isentropic ascent ahead of the shortwave trough axis above a much drier low-level tropospheric column. Clear sky or breaks in cloud cover and light or calm surface winds should contribute to the development of localized steam fog in the Black River Valley, Western Southern Tier river valleys, and the lake valleys of the Finger Lakes Region through shortly after daybreak this morning. This fog is expected to be shallow amidst scarce synoptic-scale low- level moisture. The fog will then dissipate by mid-morning, via the onset of diurnal convective mixing of the boundary layer.

The shortwave trough aloft and attendant cold front will sweep SE'ward across the eastern Great Lakes and vicinity on Thursday. The majority of the isobaric height falls aloft and large-scale forcing for ascent will be displaced well northeast of our area across QC, leaving a weakly-forced and moisture-starved cold front passage across our region. Besides the aforementioned increase in mid/upper-level cloud cover ahead of the shortwave trough axis aloft, a continued dry low-level tropospheric column is expected to permit a dry cold front passage. Behind the front, and east of the axis of a ridge at the surface and aloft that will approach from the Upper Great Lakes and vicinity, a cooler air mass will yield some increase in lake-induced instability. However, the lack of synoptic-scale low-level moisture will likely only support isolated to scattered lake- effect stratocumuli over and south of the ~16C eastern Great Lakes this afternoon into evening and then over and generally southwest of the eastern Great Lakes overnight tonight into Friday morning as mean low-level flow veers from N'erly to NE'erly. The N'erly mean low-level flow should also yield isolated to scattered upslope stratocumuli generally south of the eastern Great Lakes. All lake-effect and upslope stratocumuli are expected to be too shallow to produce precip.

Late afternoon highs are expected to reach the mid 50's to lower 60's today. Daily lows should reach mainly the upper 20's to upper 30's around daybreak today and Friday. The high pressure ridge at the surface and aloft will continue to build E'ward and settle over the Northeast United States and vicinity on Friday through this weekend. Dry weather and clear to partly cloudy sky are expected as stabilizing subsidence accompanies the ridge. Daily highs should moderate from mainly the 50's to 60F on Friday to the mid 50's to upper 60's on Sunday as our region eventually becomes located in a WAA regime along the western flank of the low-level ridge. Lows should reach mainly the mid 20's to mid 30's around daybreak Saturday and mainly the mid 20's to upper 30's around daybreak Sunday. Note: the environment should remain favorable for localized and shallow steam fog development in the Black River Valley, Western Southern Tier river valleys, and the lake valleys of the Finger Lakes Region between about 12 AM and 9 AM EDT this Friday through Sunday.

Aviation

At the surface, primarily high pressure ridging affects Western and Central NY through 06Z/Fri. However, a weak and moisture-starved cold front will sweep SE'ward through our region between ~17Z/Thurs and ~21Z/Thurs. Dry weather and primarily VFR are expected through the TAF period. Our regional surface winds trend light and variable ahead of the cold front and NW'erly to NE'erly around 5 to 10 knots behind the front.

Localized and shallow steam fog with MVFR to LIFR is expected in the Western Southern Tier river valleys, the Black River Valley, and lake valleys of the Finger Lakes Region through ~13Z/Thurs and again after ~03Z/Fri. The vertical wind profile in the lowest 2kft AGL is expected to be unfavorable for the fog to reach KJHW.

Behind the cold front, isolated to scattered lake-effect stratocumuli with bases near 3kft to 4kft AGL are expected to stream generally S'ward from the eastern Great Lakes this afternoon and evening and then generally SW'ward from those lakes overnight tonight into Friday morning. These stratocumuli may result in brief and borderline VFR/MVFR ceilings.

Outlook...

Friday Night through Monday...Dry weather and mainly VFR are expected. However, localized steam fog with MVFR to LIFR is possible in the Black River Valley, Western Southern Tier river valleys, and lake valleys of the Finger Lakes Region each morning.