GONE WITH THE WIND...
Some strange things have been going in our weather the past month thanks to a large block in the atmosphere over North America. Like a large boulder in a stream, the block has been diverting moisture, limiting storms and rainfall. In the Quad Cities, rain the past 19 days has amounted to just .01" The past 30 days just .16".
Even when we get rain, storm motions have been very erratic due to the block and weak steering currents. Typically storms travel from the southwest toward the northeast, but the past week we have seen storms travel in every conceivable direction. The Iowa Mesonet ( a great site), featured a chart to illustrate these odd storm motions by plotting a histogram of Des Moines NEXRAD storm motion tracks. The first panel shows the combined May and June climatology and the clear trend of storms storms to travel from the southwest during those months.
The panel below shows the histogram for storms this month and it is the antithesis of climatology. The dominate motion has been east/southeast. The reason for these weird storm motions is the very blocked up upper level flow that currently has weak westerly moving flow over the state.
WHERE'S THE SEVERE WEATHER?
Severe weather which is often frequent at this time of year has been non-existent. The widely scattered rains that have been in the region recently are more typical of August with slow moving afternoon thunderstorms driven by humidity and heating. The slow storm movement has even been in uncommon directions to the west and southwest. The reason is that the flow of air aloft is rather stagnant from the east and without much directional nor speed shear to support more robust thunderstorm activity. The Iowa Mesonet recently featured a chart measuring low level speed sheer between the surface and 3km aloft based on an archive of soundings from Davenport. Generally higher shear values support more robust thunderstorms by allowing the updraft and downdraft of the storms to be separated. Otherwise you get pulsey behavior as the downdraft interferes with the updraft. Anyway, the graphic shows how recent sounding values have been at the very low end of a climatological range.
What the lack of shear means is that even when we have had enough moisture and instability for thunderstorms to develop the past month, the updrafts haven't been sufficient to support severe weather. May 15-June 15th is the heart of the severe weather season in this region and it has flat lined since May 7th.
It's been 30 days since the last tornado warning was issued by the Davenport NWS service area. In the La Crosse, Wisconsin NWS service area (which includes NE Iowa) it had ben 318 days).
Up until May 7th, there had been 538 tornado and severe thunderstorm warnings issued by the DVN NWS office. Since then only 27 warnings have been issued and again none were tornado warnings. The bottom line is after setting a record pace for severe weather reports in the Quad City service area through early May, the NWS office has been essentially crickets through the heart of severe weather season.
RAIN CHANCES GOING FORWARD...
Currently a very dry air mass will sustain a pleasant forecast through Saturday. From north to south, temperatures will go from 76-81 Thursday, to 79-84 Friday, and 83-88 Saturday. Humidity levels will remain low so no issues (or rain) in this period.
Saturday night and Sunday rain prospects are in play as a well organized low pressure wave cuts across Missouri. The system is rather dynamic with a closed upper air low amplifying over Lake Michigan. More important is the track of the system as it rounds the base of high pressure over the eastern lakes. Differences remain between the GFS and EURO with regard to the northern edge of the precipitation shield. The EURO keeps the bulk of the rain over my far southern counties with minimal impacts north of I-80. The GFS shows less blocking from the Great Lakes high which allows the rain to advance further north into my area. Here's the contradiction between the two models regarding rain potential Saturday night and Sunday. As you can see there's a significant variance which leads to low confidence in rainfall placement and amounts.
The EURO
The GFS
A few light showers could even fall in the wrap around cool air Monday but amounts would be minimal, maybe a a couple tenths of an inch at best if rain can develop and reach maximum potential.
Last but not least, the blocking pattern may be breaking down, at least for a time in the long range. Here's what it looks like now at 500mb. See the cool, dry northerly component to the upper level steering currents.
Two weeks later around June 20th, the EURO had come around to the idea of a heat dome building into the SC U.S. The steering currents would then be aligned in such a way as to bring moisture into the region. A ring of fire set-up looks possible with periods of thunderstorms materializing on the northern periphery of the heat. We should be in close proximity to that potential firing zone.
I don't want to oversell the idea of rain returning in earnest, but at least in the modeling I'm seeing a pattern with more rain opportunities than in recent weeks. Hopefully we get our share.
Outside of the cool-down Sunday and Monday, readings look to be near to slightly above normal the next 10-15 days. This is the 8-14 day outlook from CPC depicting near normal precipitation! I haven't seen that for quite some time.
With that, I'm off to water the tomato plants. They look a little droopy. Roll weather...TS
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