July 18th Central Minnesota Weather Discussion

 Central Minnesota July 18th Weather Discussion

By Aidan Cera

Figure 1. Storm Prediction Center Day 2 Outlook issued 12:30PM CST.

Good afternoon central Minnesota! The main story I want to cover today is that of the severe weather risk tomorrow across portions of our area. The SPC has issued a slight risk of severe weather has been issued for Benton, Mille Lacs, Sherburne, Wright, Meeker, McLeod, Sibley, Carver and portions of Stearns, Renville and Morrison counties. All hazards of severe weather are possible including large hail (15% hatched), damaging winds (15%) and a isolated tornado or two (2%). 

Figure 2. ECMWF 250mb Wind Speed/Streamlines (kt) & MSLP Extrema (mb) 06Z run. Image courtesy of tropicaltidbits.com

From a synoptic viewpoint, a positively tilted shortwave trough is expected to advect eastward across northern Minnesota tomorrow into tomorrow night. This is expected to lead to amplified upper level flow and a appreciable increase in wind with height. Positive vorticity advection is expected ahead of this trough helping to propagate the low pressure downstream into our area, while temperature advection is expected to be quite pronounced as well. Highs are expected to climb much higher than the last several day, into the mid to upper 80s with dew points rising into the mid 60s potentially.

All of these factors together give us a fairly decent shot at scattered shower and thunderstorm activity starting late tonight and lasting through tomorrow evening. This will be a situation though where overnight activity tonight and morning activity tomorrow will influence what takes place tomorrow afternoon and evening. The more cloud cover is left over from any convection that takes place tonight means less destabilization, so cloud coverage is something to watch. 

Figure 3. NAM-3km 2-meter Dew Point (F), 10m Wind (kt), and MSLP (hPa). Image courtesy of tropicaltidbits.com. Cold fronts are personal hand analysis.

I think the main inhibitor of some areas missing out on thunderstorm activity will be cold front passage timing. It seems that once again, as has been the story of this summer, the cold front passes through a fair chunk of central Minnesota before convective initiation takes place. By early afternoon it seems that a tongue of dry air will be wrapping around the backside of the low and into central Minnesota, especially the northern and western portions of central Minnesota due to the cold front already passing through by around 1-2 PM. 

Given that peak heating usually doesn't take place till after this, and that the dew point will decrease sharply after the cold fronts passage, it seems unlikely that areas west and north of St Cloud will see anything at all. In fact, I don't expect much in terms of thunderstorm activity till the cold front is passing closer to the Twin Cities and approaching the I-35 corridor. Areas south of Willmar to the Twin Cities still have a fair chance of thunderstorm however, but I think our north and western counties will go dry again if this model trend holds. It all depends on when the cold front comes through though. If it comes through sooner fewer areas see rain. but if it comes through slower, more areas have a chance to see rain. Given that cold fronts have tended to come through sooner this summer I am inclined to lean more toward that direction but will be watching the model trend and real time data to see if any changes take place. 

Not only is timing as issue, but mesoscale models hint that a inversion may be present around 850 mb. This may act to stunt development which could be why models are so unenthusiastic about more widespread thunderstorm development tomorrow. The cold front passing through later would give the air more time to mix and break through the cap, so again, later cold front passage means more thunderstorm coverage potential. 

If any thunderstorms are able to fire up, some of them will likely become severe given the increase with wind speed with height which will help tilt updrafts to sustain thunderstorm growth for a longer period of time. Also prior to cold front passage, dew points and temperature profiles with fairly steep lapse rates below a weak thermal inversion around 850 mb will provide ample instability with CAPE values well above 1,000 J/kg and perhaps reaching into the 2,000 J/kg range. Large hail and damaging winds seem to be the main threats but an isolated tornado can not be ruled out in areas where winds are more strongly veering. 

By tomorrow night dry air will be wrapping in and storm chances come to an end for most of central Minnesota except for our far northern counties that may see some wrap around shower and storm activity associated with the lingering area of low pressure to the north.

The rest of the forecast period looks very dry and increasingly warmer after Thursday as each day goes on as models continue to show the development of a "death ridge" across the High Plains which could be around for quite some time and may lead to the return of 90Fs here by early next week with very little in terms of rain chances. It looks like this upcoming Thursday will be our coolest day, with highs in the upper 70Fs to around 80F thanks to the cold front passing through tomorrow but again, enjoy the milder weather while it lasts because this ridge of high pressure that models bring about doesn't seem to be playing around. More details are to come in future updates.