textproduct: Columbia

WHAT HAS CHANGED

Little change in overall thinking with a conditional severe threat Sunday afternoon and a more widespread threat of severe weather including tornadoes on Monday with a strong cold front. 18z aviation forecast updated.

KEY MESSAGES

- 1. Scattered strong thunderstorms possible on Sunday afternoon/evening with a conditional tornado threat. - 2. A dynamic weather system will approach on Monday bringing potential severe thunderstorms with all hazards possible and strong, non-thunderstorm wind gusts expected.

- 3: Potential for a hard freeze Tuesday night into early Wednesday.

DISCUSSION

KEY MESSAGE 1: Scattered strong thunderstorms possible on Sunday afternoon/evening with a conditional tornado threat.

The beginning of an active weather period starts on Sunday as a very dynamic and intensifying 500mb trough ejects from the central Rockies into the Plains and Mid Mississippi Valley by Sunday evening. This will result in the development of a strong surface low over the upper Midwest that deepens into the Great Lakes and pushes a strong cold front into the Ohio Valley southward into the lower Mississippi Valley. Ahead of this system, increasing southeasterly flow off the Atlantic will result in low level moisture advection across the forecast area with surface dewpoints rising into the lower to possibly mid 60s. Hi-res guidance forecast soundings get pretty interesting after 18z with SBCAPE values in the 500-1000 J/kg range with sufficient 0-6km shear around 40-45 knots. Hodographs show increasing low level curvature during the afternoon/eve hours with a peak during the 21z-02z time frame with 0-1km SRH values over 100 m2s2 and 0-3km SRH values approaching 200 m2s2 so there will be a conditional tornado threat with storms that develop. Hi-res guidance overall shows mainly scattered convection but the forecast environment suggests the risk for marginal severe weather if the instability is realized and SPC has the area outlooked in a marginal risk. Damaging wind gusts is likely the primary hazard for any severe storms but the tornado and, to a lower degree, hail risk is present as well.

KEY MESSAGE 2: A dynamic weather system will approach on Monday bringing potential severe thunderstorms with all hazards possible and strong, non-thunderstorm wind gusts expected.

The aforementioned dynamic upper trough and surface low will be moving into the Great Lakes region late Sunday night into Monday morning pushing a strong cold front towards the forecast area. Significant height falls are expected to overspread the region atop a moist and modestly unstable boundary layer (CAPE values around 1000 J/kg and mid 60s dewpoints). Deep layer 0-6km shear is forecast to be in excess of 50 knots and hodographs will be favorable for strong low level shear with 0-1km SRH values in excess of 200 m2s2.

The cold front itself will be quite strong with a cross front temp gradient of 20-25 degrees providing significant low level forcing coinciding with the strong upper forcing of the height falls and approaching shortwave energy. This should support a potential widespread severe weather event across the region with a QLCS line expected to traverse the forecast area with potential isolated discrete cells ahead of it. QLCS tornadoes appear possible along with the potential for stronger tornadoes in any discrete cells that may form ahead of the line. This has resulted in the SPC enhanced risk across the forecast area with damaging winds as the primary threat but as mentioned tornadoes will be possible as well as large hail. The timing for this has not changed much and still thinking the 8am to 2pm time frame is the main window for severe weather with the highest confidence being in the central and eastern Midlands as instability increases through the late morning hours Monday.

In addition to the severe threat, the increased pressure gradient with this system will be strong and provide non- thunderstorm wind gust potential in the 30-35+ mph ahead of the front with potentially stronger gusts behind the front when strong cold air advection commences tapping into 45-55 knot 850mb winds. The ECE EFI continues to show a signal for anomalously strong winds and this strong wind gust potential may result in the need for a wind advisory.

KEY MESSAGE 3: Potential for a hard freeze Tuesday night into early Wednesday.

Behind Monday's front, arctic high pressure and dry air are expected to move into the region, causing temperatures to plunge to well below average Monday night and especially Tuesday night. With the arctic airmass in place, overnight lows Monday could drop to around the freezing mark. However, as the front continues to be somewhat near the region, winds are forecast to remain elevated Monday night, resulting in some uncertainty with how cold temperatures will get. Tuesday night is a different story as high pressure is forecast to be essentially overhead, leading to efficient radiational cooling conditions. With the efficient radiational cooling in place, confidence is high that temperatures in most locations will drop below freezing. Confidence also continues to increase in the potential for a hard freeze, with greatest chances in the northwestern portions of the forecast area. Assuming these trends continue, Frost/Freeze products will likely be needed Tuesday night, and maybe Monday night.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Outside of River Fog, VFR Conditions Likely Persist through the TAF Period....

VFR conditions expected through much of the TAF period, with potential for MVFR conditions late in the period.

High pressure over the region into tonight brings mostly clear skies and light winds through much of this TAF period. Increasing moisture brings lower clouds around 12z or so to the terminals, with ceilings potentially dropping to MVFR at times from 12-18z. There is also a small chance for a shower or thunderstorm prior to 18z at the terminals, with a slightly higher chance at AGS/DNL than the other terminals. That said, chances remain too low to put in the TAF at this time. East to southeast winds are also expected to pick up to around 10 kts after about 12z or 13z.

EXTENDED AVIATION OUTLOOK...Restrictions are possible on Sunday afternoon and evening from scattered showers and thunderstorms. A cold front moving through on Monday likely brings restrictions in showers and/or thunderstorms, both ahead of and behind the boundary. A return to VFR conditions is then anticipated for Tuesday onward.

CAE WATCHES/WARNINGS/ADVISORIES

SC...None. GA...None.