Severe Storm Outlook Narrative (AC)
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
940 ACUS01 KWNS 201248 SWODY1 SPC AC 201247 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0747 AM CDT Mon May 20 2024 Valid 201300Z - 211200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF NORTHEASTERN COLORADO...CENTRAL/SOUTHWESTERN NEBRASKA...AND NORTHWESTERN KANSAS... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE ENHANCED RISK...AND ALSO OVER PARTS OF THE SOUTHWESTERN LAKE MICHIGAN VICINITY... ...SUMMARY... The greatest severe-thunderstorm threat today appears to be over parts of the central Plains, with large to very large hail, severe gusts (some 75 mph or greater), and a few tornadoes all possible. ...Synopsis... Mid/upper-level troughing over the western CONUS is expected to shift slowly eastward through the period, as an associated, strong shortwave trough pivots southeastward from the interior Northwest to WY/UT. A southern-stream trough -- apparent in moisture-channel imagery over the Pacific west of southern CA/northern Baja -- should eject northeastward and reach AZ by 00Z, then the central High Plains by 12Z tomorrow. A broad fetch of difluent flow aloft will extend downstream across the Great Plains States, with westerlies over OK and southwesterlies from CO to the Upper Midwest. Meanwhile, a positively tilted trough will drift southeastward over the northern/central FL Peninsula. The 11Z surface map showed a low in the FSD/SUX region, with quasistationary front southwestward to another low over southwestern KS, then arching westward into southeastern CO. Cyclogenesis should occur along the western boundary today, over eastern CO, as well as tonight along a dryline over the northern TX/western OK Panhandles. The lows should combine along the front, over western KS, by the end of the period. A warm front was drawn from the Siouxland low across southeastern IA, and should move northeastward over northern IL today. ...Central Plains... Thunderstorm development is expected by late afternoon, initially over higher terrain and in post-frontal convergence/upslope-lift zones near the Palmer Divide and/or Cheyenne Ridge. Some of this activity should move east-northeastward over northeastern CO and adjoining portions of NE, with both supercells and upscale- aggregating storm cluster(s) expected. Multiple rounds of convection may cross the region in and near the "Enhanced" area through this evening. Large to very large hail and a few tornadoes are possible from any sustained supercells. Severe gusts may occur from supercell downdrafts, but should be more common once cells evolve to clusters and cold pools combine. Some significant (65+ kt) gusts are possible. Activity should move/grow into a corridor of favorable instability across and south of the front in NE, with more convection developing near the front tonight. Isolated development also is possible near the dryline over eastern CO/western KS, but should be shorter-lived. Large-scale ascent aloft (ahead of the approaching Intermountain West trough) and low-level lift each should increase across the outlook area throughout the afternoon and into the evening. The boundary layer also should destabilize (preferentially more at higher altitudes) diurnally -- though the influence of moisture will contribute to increasing CAPE eastward across CO to the southwestern NE/northwestern KS area. The airmass-recovery process over that region will continue for most of the day, and the threat may develop and peak later in the day/evening than usual for this region. Relatively moisture-rich boundary-layer trajectories (still bearing the imprint of some upstream convective modification) may not advect into the area until late afternoon or evening. Still, with steep midlevel lapse rates attributable to the EML, MLCAPE values commonly around 2000 J/kg should develop, amidst 40-50 kt effective-shear magnitudes. Long hodographs will support the hail threat. Nocturnal boundary-layer stabilization and delayed airmass recovery with eastward extent make probabilities more conditional into eastern NE, IA and southern MN. ...Upper Great Lakes region... A prominent MCV -- produced by last evening`s destructive thunderstorm complex over KS -- is apparent in composite radar imagery over IA. This feature should move northeastward across the DBQ ares and southern WI today, then over central Lake Michigan and the northern Lower Peninsula through tonight. As the associated field of strengthening low/middle-level ascent and mesoscale shear enhancement spreads over a destabilizing boundary layer in northern IL and northwestern IN, one or more episodes of thunderstorms should develop and move across the area from later this morning through late afternoon. Pockets of heating in cloud breaks, along with surface dewpoints in the low/mid 60s F, should support MLCAPE in the 1500-2500 J/kg range. Ambient deep shear is progged to be modest, though this may be underrepresented by synoptic models as often seen east through southeast of a well-formed MCV. Organized multicells/ clusters, and perhaps a few supercells, are possible. Damaging gusts and isolated severe hail are the main concerns. Some convection-allowing progs depict either: 1. A rejuvenated version of the IL convection later over Lower MI, past the impediment of relatively cold lake waters, or 2. Additional development in the Lower Peninsula as the MCV approaches Lake Michigan and the associated ascent field spreads overhead. While either scenario cannot be ruled out, the timing would place the activity into marginally surface-based effective-inflow parcels, amid decreasing instability eastward and with time. Therefore, the unconditional severe probabilities will be kept marginal over Lower MI for now, though even slightly more favorable mesoscale trends or more-consistent progs may compel an upgrade to wind probabilities for parts of the area. ...Southeastern FL... Scattered multicell thunderstorms are expected to form over mainly eastern parts of the region today, especially south of a weak cold front, along the Atlantic sea breeze and on outflow boundaries from initial-stage convection. Isolated severe gusts and hail are possible. Surface dewpoints will remain in the 70s F, acting in concert with strong heating to weaken MLCINH, and offset modest midlevel lapse rates enough for around 1500-2500 J/kg MLCAPE. The main limiting factor will be modest shear. Mid/upper-level flow will not be as strong as previous days, limiting deep shear (effective-shear magnitudes less than 30 kt), while weak winds in the lowest km keep hodographs fairly small. ..Edwards/Goss.. 05/20/2024 $$