Area Forecast Discussion · National Weather Service Omaha (OAX) · issued Thursday, July 16 at 5:37 PM CDT.
Messages
Isolated showers, even one or two thunderstorms will be possible this afternoon.
Heat builds into the weekend, with heat index values near 100 to 105 at times from Friday through Monday.Heat index rising above 105 along the Nebraska- South Dakota border. Actual temperatures around 100 for Monday.
Cooler temperatures Tuesday through Thursday, dropping back into the 80s and 90s.
15-30% chance of thunderstorms in northeast Nebraska on Sunday night, then dry conditions until Thursday, when shower and thunderstorms chances increase to 30-40%.
Discussion
The latest satellite imagery has a more robust cumulus field today, as a low level stream of greater theta-e air advects from the lower Mississippi Valley. Diabatic heating and mixing is tapping into low-mid level instability, which is further amplified by a weak, meso-scale (?) swirl located over northeast Colorado undercutting the larger upper level ridge. The result are isolated showers popping up where local thermodynamic forcing is the best. Any showers/sprinkles will dissipate with the setting sun. Under clear skies, tonight will be a little muggier with increasing dewpoints, with lows in the lower to middle 70s in most locations.
Expect 2-5 degree bump in temperatures for Friday, with H8 thermal ridge oriented from northeast Colorado into western Minnesota, ahead of a weak frontal boundary. Compressional heating and mixing will allow temperatures to rise into the low 100s along the NE-SD border, with lower to middle 90s elsewhere. There's a 45-50% chance of a heat advisory being hoisted for counties along the Nebraska-South Dakota border for Friday. The boundary moves into northern Nebraska and Iowa for Saturday, bringing minor cooling north the boundary, but the low-level and H8 thermal ridge then orients south of this boundary. Thermal mixing will not be as efficient surface-H8 winds becoming more southeasterly, thus temperatures about the same or even a degree or two cooler, with heat indices in the 94-104 range.
These days continue to look the hottest. The low level thermal ridge remains in place, and simultaneously H5 heights increase. Low-level flow veers to more southerly on Sunday, and more southwesterly on Monday. Ensemble Extreme Forecast tables denoting temperatures in the 90-95% percentile, with shift of tails of 1,for for the maximum temperatures both days, and the minimum temperature for Monday morning. So, what does this mean, a more extreme event than 30 year climate record. We're still a 4-5 days out, but at time there is a greater probability of areawide heat headlines for these days.
The fly in the ointment for Monday will be the expanse of nighttime convection Sunday night into early Monday. A mid- level shortwave moves across the northern U.S./southern CP plains in tandem with a pronounced jet streak With H7 temperatures at or above 12C and H5 temperatures 0-5C, it will be rather warm for any hail development, though collapsing storms would have wind potential.
Relief form the heat comes as H5 ridge flattens in the wake of the aforementioned shortwave/jet streak combo, driving a cooler front into southern Kansas. This will reduce temperatures about 5 degrees for Tuesday, with further cooling Wednesday and Thursday with more earnest surface to H8 CAA. We may see nighttime convection Wednesday night into Thursday, as storms develop in the lee of the southern Rockies and then are pulled eastward by a weak shortwave moving through the flattened H5 ridge.
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