Referring to the reflectivity/MESH post a few minutes ago, OUN WRF showed some signals of an increase in the severe risk just as storms achieved 60 + dBz cores. Attached below one can see an overlap of instantaneous updraft helicity (green) and surface max hourly column hail (brown). For most of the period until now, both parameters were persistently separated, which is shown below in the first image. Those storms had a pulsating character with relatively weak cores.

It was interesting to see that betimes, OUNWRF produced stronger signals with a a better overlap of both parameters. Eventually storms began to intensify just in this area, finally producing our strong thunderstorms. OUNWRF gave some lead timing and increased confidence to the forecaster that an increase of thunderstorm’s strength could be expected.

Helge

Monitoring the potential for convective development across W Minnesota / SE South Dakota, and across central Nebraska.

Shortwave trough currently progressing east across S Saskatchewan into E Ontario providing support for some initial development across the Canadian provinces, as seen in IR / VIS / CTC data. Large-scale lift aloft, in association with eastward progressing frontal boundary is supporting a line of agitated cumulus extending south into NW Minnesota. Nearcast difference CAPE product indicates an axis of weak-to-moderate instability (500-1000 J/kg) in front of this boundary extending south into NE and IA. WRF simulated IR fails to develop much of any activity across N Minnesota, keeping all activity developing after dark across the UP of Michigan. Farther south and west, it shows better development across Iowa and Nebraska around 00Z. However, considering best large-scale forcing exists closer to the Canadian border, and model fields were generally overestimating surface dew points farther south (forecast >60 versus AOB lower 50s being observed), thought is that best potential may exist closer to the international border over the next few hours.

If storms can develop, dry low levels and steep lapse rates support a high wind threat from higher based storms. Better wind fields farther north (0-6 km shear >50 kts) and slightly cooler temps also support an enhanced threat for marginal severe hail closer to the Canadian border.

Picca

Isolated to scattered unorganized thunderstorms currently over beautiful West Texas will likely increase in coverage through the afternoon and may congeal into one or more MCS by 00z as the storms move east into increasing 0-6 km bulk shear (from 30-40 kt) depected by the RAP13 and increasing MLCAPE (800-1500 J/kg) suggested by LAPS.  The image below shows the LAPS surface-based CAPE…depicting the area of highest instability at present where storms will be favored to develop.

  The 18 UTC OUNWRF model reflectivity depicts the expected scenario.  The first image shows the model composite reflectivity forecast at 2030Z, and the second is for 0200 Z, showing the change in mode from multicells to MCS.  The main hazards will most likely transition from isolated large hail reports to more damaging wind reports as the storms become better organized.  The MCS may weaken with the loss of daytime heating and become more elevated after 02z with a diminishing wind and hail threat.

Placed beneath the left exit region of an approaching mid/upper jet from the SW, scattered storms evolved in a weakly sheared environment. Latest reflectivity scans reveal a few 60 dBz scans with strongest cores and MRMS MESH pointing to some small hail…still sub-severe. With SBCAPE already exceeding 1500 J/kg in many places, the overall hail and wind gust risk constantly increases during the following hours…however scattered initiation likely causes messy clustering of storms with decreasing diabatic heating. The focus for more severe storms will be east of the Midland area, when storms finally evolve in a stronger shear environment. We’ll look for current storms to send outflow boundaries to the east for initiation in SPC’s SLGT risk area next to forced initiation as the upper trough further approaches.

Radar reflectivity with MESH atop.

Helge

Gabe, the OUNWRF PI, showed us an interesting way to monitor a few model output fields simultaneously.  The display here includes the model derived radar reflectivity as a background image, with the green contours showing updraft helicity and the salmon contours the maximum hourly column hail.  The display here is consistent with the expected mode of convection over the next few hours…namely, pulse storms or multicell clusters.  With marginal bulk shear values, we aren’t expected a lot of deeply rotating storms, but there will be a large hail and/or damaging wind threat with some of the stronger cores.  I will continue to watch this display through the evening to see how it all pans out.

CL

This is a image of LAPS_OUN_3 km run from 15 UTC 05/14/13 for 18 UTC compared to actual 0.5 Degree reflectivity from KMAF at 18 UTC.  The LAPS data were nearly dead on with the location, coverage, intensity, and storm mode with the storms southwest of Midland with perhaps a slight underestimation of activity near the NM/TX border.

This is a image of the OUNWRF 16 UTC run 05/14/13 for 1845 UTC 1 km AGL Derived Radar Reflectivity compared to 0.5 Degree Reflectivity from KMAF at 1845 UTC.  They compared well with the OUNWRF doing a fine job with the location and storm mode (isolated cells/multicell clusters) near and just southwest of Midland.  The OUNWRF underestimated unorganized convection across SE NM.  Michael Scotten

Models have been trending upward for severe chances in southwest Texas for today and the most recent day 1 outlook from SPC has moved from a “see text” to an actual “slight risk” for the region.  The combination of a dryline circulation, forced lifting and an upper-level jet streak will increase the probability of severe weather compared to the western great lakes region we had previously targeted which is still dealing with a lack of moisture and a strong cap.

Possible CWAs:  San Angelo, Austin/San Antonio, Midland

K. Calhoun, Week 2 Coordinator

The 2.5 km variational LAPS did a relatively good job at resolving various boundaries that became apparent through the afternoon. Looking at the LAPS surface dew point and wind fields across the northern Rockies around 22Z…higher terrain locations had lower dew points in the 20s and 30s…with the lower terrain locations in the valleys and plains of northern Montana showing dew points in the 40s and 50s. In addition…locations in the wake of convective activity across northeast Oregon and points to the northeast both showed higher dew points and outflow boundary-like signatures in the wind field. These both matched reasonably well with the METAR observations. The model also did well with identifying the convergence boundary in the wind fields stretching from eastern Oregon through central Montana. Mazur

Location: Montana. Threat: severe hail/high winds. Soundings suggest potential for thermodynamically-driven downbursts over mountainous terrain. Question: will cloudtop cooling (CTC) rate be applicable to hail cores that could generate wet/dry microbursts?

CTC rates have been noted to have problems in mountainous regions such as the Rockies and even Appalachians. The rates were at or below 10C/15 minutes on the first few updrafts and only one storm briefly (one volume scan) reached 50 dbz through 20UTC. A storm entering Idaho County showed cooling rates of 12.6C/15 minutes at 2015 UTC, but this storm did not intensify until after 2130 UTC. This storm was not assigned a CTC rate after 2015 UTC because of masking by cirrus blowoff. Another storm that formed on the forward flank of this storm became marginally severe but was also never identified due to cirrus.

Possible sampling issues were noted in the MRMS sfc height of 60 dbz above -20C. The levels showed big scan to scan variability which may have been a function of distance from KMSO and sparse coverage of surrounding radars. As this storm got closer to the RDA, the scan to scan variability decreased.

SPC issued a severe thunderstorm watch at 21UTC with the main threats being large hail and high winds, with a secondary threat of a tornado.

– Ostuno