North Georgia Weather Blog

SCCA National Championship - Morning Weather Briefing for Saturday, September 1

9/1/2018

NWS Omaha Forecast Discussion

Thunderstorm chances/intensity/timing/rain amounts will provide the main forecast challenges through Monday. Pattern early this morning is not too different than last night, and it will remain similar the next few days.

Will keep some chance of storms around this morning, but overall confidence on what will happen this afternoon is not high. This is because of a fairly wide spread of what the short range, high resolution convection allowing models (CAMS) are indicating. In regards to the 00z runs of the GFS and NAM, the GFS seemed to handle the activity last evening and this morning better than the NAM so will give it a bit more weight for this evening/tonight.

There is potential for heavy rain in the forecast area tonight, but exactly where is yet to be seen. There will be a stalled front in the area and models generally show PW around or over 2 inches. Will keep thunderstorm chances tonight in the 30 to 70 percent range, lowest near the SD and MO border areas. Rainfall rates could be high enough to cause flooding, but will not issue a Flash Flood Watch at this time.

Cloud cover and precipitation held most temperatures down on Friday. Most areas today should see highs in the lower or mid 80's, with mid 80's to near 90 possible for parts of southeast NE and southwest IA. (See images below)

Weather Prediction Center - Excessive Rainfall Discussion

Convection may maintain in some capacity through the day, but the combination of the wave flattening as it moves east, the upper jet streak progressing east, and weakening of the low level inflow, should result in less organization of convection with time today (or at least a shrinkage in size of any organized activity). However by later this afternoon into the evening hours should see convection once again begin to expand in coverage from KS/NE northeastward into MO/IA/IL.

In the mid and upper levels should have another shortwave pushing east, with another strengthening jet streak to the north increasing upper level diffluence. In the lower levels will have a few west to east boundaries helping focus convection, with outflow from this mornings activity potentially a feature in play. Will also have another round of increased southerly 850mb moisture transport... with this axis a bit broader this time. This should favor a wider west to east zone of convective activity... and while this moisture transport axis may try to lift northward with time... cells back building to the south into the enhanced flow may help keep the convective axis nearly stationary for a period of time... resulting in a training threat.

Thus... looks like another good setup for heavy rainfall and potential flash flooding this evening into the overnight hours. Given the multiple rounds at play... some potential that a portion of the Slight Risk may need to be upgraded at some point if there is sufficient overlap geographically with the multiple rounds. However will wait and see what falls out of this mornings activity... and wait and see what the 12z HREF/HRRR depict for the second round of activity... before considering any upgrade. Models are in pretty good agreement on the overall setup... and their QPF axis are not all that different either. Although still some latitudinal and timing differences noted. The Slight Risk follow close to the axis depicted by the 12z HREF and HRRR runs.