Flash Flood Guidance
Issued by NWS
Issued by NWS
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
341 AWUS01 KWNH 220254 FFGMPD WIZ000-MNZ000-IAZ000-SDZ000-220845- Mesoscale Precipitation Discussion 0485 NWS Weather Prediction Center College Park MD 1053 PM EDT Fri Jun 21 2024 Areas affected...Southeast SDAK...Southern MN...Northern IA...Western Wi... Concerning...Heavy rainfall...Flash flooding likely Valid 220255Z - 220845Z SUMMARY...Considerable flash flooding event likely to broaden in coverage (S MN/N IA) through overnight periods. Embedded within locally signficant, life-threatening flash flooding likely across portions of southeast SDAK/northwest IA. Additional 3-6" totals are expected. DISCUSSION...GOES-E 10.3um EIR already show a developing MCS with broadening canopy over SE SDAK/SW MN/NW IA with tops cooling below -65C indicative of the unstable air over 2000 J/kg MLCAPE. The synoptic environment continues to support further strengthening as well as slow eastward advancement with impressive dual jet ascent outflow channels to spur strengthening surface and low level wind flow/flux into the developing MCS. This is strengthening a surface low in far northeast NEB along a stubborn west-east stationary front that extends north of LRJ-CAV-OLZ to the WI/IL border. Surface to 850mb flow is nearly southerly, orthogonal to the boundary with VWP 35-40kts of LLJ extending nearly the width of Iowa. This is transporting high theta-E air with surface Tds in the low to upper 70s that remains in the 60s though 850mb...allowing for AoA 2" total PWats and solid flux to support 2"/hr rain rates, occasionally increasing to 2.5-3"/hr with storm scale/isallobaric convergence centers over the coming hours. Though there is a subtle dual jet feature in the upper-jet to support the stronger divergence, the main steering flow becomes fairly unidirectional through 700-300mb just a bit north of due east to allow for a prolonged length of training convection. While the flow should allow for 25-30kts of eastward motions this will allow for hours of training with 3-6" totals likely to occur across S MN/Northern IA. This area, especially over far SE SDAK, and of the Iowa border is nearly fully saturated with NASA SPoRT 0-40cm saturation ratios of 80-90+%, so nearly all this will be run-off or support rapid inundation, this will likely lead to a broad area of considerable flash flooding with probable embedded areas of locally significant, life threatening flash flooding throughout the overnight period. Areas in north-central Iowa and southeast MN will see saturation values rise and coverage of flash flooding incidents are likely to increase, but be a bit less in coverage/magnitude that points further west. Of note, but of lower certainty and toward the end of the valid period (06-09z) The larger scale synoptic trough is exiting the northern Central High Plains into W SDak, this will once again bring mid to upper level height-falls and result in some backing of the low level flow and eastward propagation of the surface low and frontal zone. While current convection is exhausting the bulk of instability, backed flow as well as steepening lapse rates may maintain instability to generate upstream thunderstorm development across south-central to south-east SDAK, or more likely allow for cells to persist/back-build in the vicinity of the surface low/MCV increasing duration of moderate precipitation over these already considerably flooded areas; so local trends will need to be closely monitored for this potential uptick that may result in further 1-3"+, but again, remains highly conditional/uncertain. Gallina ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...ARX...DMX...FSD...MPX... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 44569479 44529242 44069068 42799066 42649262 42269534 42709680 43089777 43629793 44279664