Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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
314 FXUS63 KDMX 021750 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1250 PM CDT Sun Jun 2 2024 ...Updated for the 18z Aviation Discussion... .KEY MESSAGES... - Today`s highest concern for severe weather is over northern Iowa late this afternoon/evening and then western half of the state late this evening and overnight. Damaging wind gusts and possibly localized flash flooding the main concerns. - Storms later Tuesday into Tuesday evening bring a higher chance of severe storms and possible locally heavy rainfall - River rises are likely through the end of next week due to rainfall && .DISCUSSION... Issued at 337 AM CDT Sun Jun 2 2024 Of first concern this morning is the fog potential, which so far has remained largely just east of the area with just Forest City/FXY reporting reduced visibility as low as a quarter at times. GOES-East Nighttime Microphysics RGB shows fog signal over portions of far eastern Iowa that are then masked by high clouds moving eastward across the state. High resolution deterministic guidance and 0z HREF continues to place the fog up against our forecast area along Highway 63 from Ottumwa to Waterloo. There may be a higher chance over northern Iowa looking at current satellite as the high clouds have not been as prevalent there and may allow for the best radiational cooling setup. Guidance is showing this as well and will see if other sites join and if severity warrants, a dense fog advisory is possible yet this morning. Any fog will dissipate a few hours after sunrise and at the same time we will be watching convection arrive from Nebraska. Models have not done outstanding with initializing overnight, but the latest RAP and to a lesser degree the HRRR and RRFS seem to have the best handle. These would support convection diminishing in intensity and spatial extent as it moves into our forecast area along roughly Highway 71 a little after sunrise. There is greater uncertainty in what happens through the remainder of the daylight hours with run to run inconsistency in the high resolution models including the HRRR not lending itself to any confidence building. Most high resolution guidance suggests isolated festering of showers or storms as they move slowly eastward through early afternoon with this most probable over northern Iowa. There still remains a signal for severe convection late this afternoon into this evening over northern Iowa with the 6z HRRR coming in rather aggressive, which has a companion in the FV3 with also an uptick in the 0z HREF paintball reflectivity. Both models show wind gusts to around 50 knots and the environment would be supportive of large hail as well. These models also paint a concerning picture for isolated flash flooding as an initial round moves through followed by a west to east line of storms over the same area before the overnight convection arrives. More on this in a moment. As just alluded to, attention will also be west over Nebraska as storms develop later this afternoon and grow upscale into a mesoscale convective system that will be cooking with severe, possibly significant, wind gusts. While it does look like these storms lose steam as they approach the Missouri River, the severe risk may not fully subside as it enters western Iowa and our western forecast area. High resolution guidance has shown wind gusts to near 50 knots, though latest is keeping that between 40 and under 50 knots. 0-3km bulk shear will be around 30 knots and oriented favorably to the line and any surges may allow for possible QLCS tornadogenesis. Back to the flash flooding concerns tonight. The 1/12z and 2/00z HREF localized probability matched mean has a bulls-area between Highways 20 and 3 around Storm Lake with 5+ inches in 24 hours. This is being driven by the FV3, which shows 3 hour rainfall of 5+ inches and that is 2.5 times the 3 hour flash flood guidance (!). Of course, the FV3 tends to be a more aggressive models, but seeing a signal in the HRRR of 3-5 inches in roughly the same area with the GFS showing 4-6 inches along and north of Highway 3. Since this is the first run of the extended HRRR to show such and there is quite a bit of uncertainty given whether or not the late afternoon/evening convection plays out and if the evolution described above would come to fruition, will not jump into a flash flood watch just yet. The bulk of showers and storms will be pushing out Monday morning as the subtle shortwave moves into the Great Lakes region. There may be a short period of dry conditions across much of central Iowa; however, guidance does show redevelopment of showers and festering storms in the afternoon most prevalent over southern Iowa where weak low level thermal lift will remain. These may maintain into the night in a more isolated fashion as a lead shortwave trough moves through Missouri and may help to provide some nearby forcing. The main longwave trough will be advancing quickly from the British Columbia province and Northwestern States Monday night as it dips into the Dakotas and negatively tilts on Tuesday. Surface low pressure will stay over the Canadian prairies of Saskatchewan and Manitoba with a cold front extending southward to Iowa. Ahead of this front, instability will grow with strong MUCAPE over 2500 J/kg and there may be a little better area overlap of this instability with favorable deep layer shear for severe storms. SPC highlights the area with a marginal on their day 3 outlook with the Colorado State`s machine learning random forest outlook showing probabilities over the state as well. As for the flash flooding threat, the environment will have precipitable water values over 1.75 inches and may be pushing to near 2 inches with very favorable warm cloud depths of 3500 to near 4000m for efficient warm rainfall processes. Soundings continue to support locally heavy rainfall being moisture laden through much of the column with NAEFS specific humidity at 925mb at the 99.5th percentile over portions of the state. Any storms earlier in the day will be more favorable for heavy rainfall from the standpoint of weaker 850-300mb flow while later in the day the flow will be more progressive. Rainfall that occurs between now (Sunday) and Tuesday may also prime an area and be more susceptible to flash flooding. For now, WPC has highlighted our forecast area with a slight risk (2 out of 4) of excessive rainfall. Rainfall totals from coarser global models in 6 hours may be 1 to 2 inches, though likely will see some higher amounts as we enter the high resolution guidance window. This is also true in the ensemble data world with 25th percentile between a third and a half an inch to 75th percentile of 1 to 1.5 inches. The rainfall from this weekend through Tuesday night will lead to river rises. The 1/12z experimental Hydrologic Ensemble Forecasting Service (HEFS) data shows a few locations (HBTI4, CEDI4) with a 50% chance of exceeding flood stage by mid to late week with a 30% chance of several points within the Des Moines, Raccoon, and Cedar Basins exceeding flood stage. Behind the cold front that moves through Tuesday night, drier air will arrive along with seasonal temperatures in the 70s to around 80 degrees for the remainder of the week. The longwave trough will wrap up into a sprawling closed low over the Great Lakes region from midweek into next weekend. This will keep the flow over the region northwesterly. Forecast for now is dry through the end of the week, but do note in the ensemble data some light QPF from various members. && .AVIATION /18Z TAFS THROUGH 18Z MONDAY/... Issued at 1249 PM CDT Sun Jun 2 2024 VFR conditions were in place across the forecast area at midday with varied cloud coverage and heights, and weak showers approaching KDSM. Confidence in how things progress later in the period is low however. There is certainly some potential for additional storms, especially tonight, but timing and location details are non- descript at this time due to variable guidance. The most likely scenario will be a period of storms west and central, with less confidence east, which has been reflected in a period of -TSRA or VCSH for the time being. MVFR or less conditions are possible with any stronger storms, but as mentioned confidence is too low for inclusion. && .DMX WATCHES/WARNINGS/ADVISORIES...None. && $$ DISCUSSION...Ansorge AVIATION...Small