Monday Trough to Bring Shower, Isolated T-Storm Chance

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After our first bout of widespread 90s during the workweek, this weekend has been much cooler in comparison due to a trough/surface low off the Pacific Northwest coast nudging our ridge east allowing for a deeper penetrating westerly flow into the state.

Trough will deepen a bit and dig southward into California through the day Monday, with a vort-max/shortwave forming offshore along the base of the trough. As the trough slides overhead Monday afternoon, it continues to strengthen to the point a cutoff upper low forms within the base of the trough. The trough will be positively tilted, which isn’t as favorable of a tilt compared to negative for all things precipitation and convection. Nevertheless, an offshore cutoff low streaming moisture eastward ahead of the incoming trough/shortwave, some of which may bank up in the valley with precipitable water values in the .5 – .7″ range, which even then isn’t too noticeable. Dew points only look to be in the 40s as well, so moisture will be lacking.

18z 12km NAM's forecast 500mb heights and relative vorticity valid Monday evening.

18z 12km NAM’s forecast 500mb heights and relative vorticity valid Monday evening.

At 500mb, temperatures cool to around -20c-ish, and down at the surface in the valley, partly cloudy to sunny skies in the morning through early to mid-afternoon will allow temperatures to jump into the upper 60s to low 70s. These temperatures beneath these moderately cool upper-level temperatures will support some very steep low-level lapse rates, and with the modest instability available some surface-based CAPEs in the 100 – 300 j/kg range and a pocket of mixed-layer CAPE in the 100 – 200 j/kg range from Yuba city to Sacramento in the late afternoon and early evening directly under the vort-max/developing upper-low. Short-range mesoscale models such as the 12km and 4km NAM(s), SPC WRF, and local (Sacramento) NWS-run WRF indicate shower and possible isolated thunderstorm development in the coastal mountains and west side of the valley after 1pm or so, continuing/lingering through around 10pm – 12am in the valley, and into the early morning hours of Tuesday in the foothills and westerns slope of the sierra as the trough slowly ejects east.

May 4th 2014

The instability currently modeled, while not significant even based on California standards, would be sufficient for some elevated thunderstorms in combination with relatively high LCLs. A lack of mid/upper-level winds in the base of the trough also means little shear, thus no big severe threat is currently anticipated, however before evening moisture recovery, before sunset, forecast soundings indicate a relatively dry sub-cloud layer/inverted-v profiles, thus any thunderstorms that develop could produce some gusty winds and/or stronger downbursts given potential DCAPEs in the 200 – 300 j/kg range.

May 4th Threats

Things dry out after Tuesday as ridging takes over, however a bit of a westerly flow continues through at least the rest of the work week, keeping us out of the heat. By the weekend into next week though, much stronger ridging takes over the whole western U.S. coast, and by that time we could be back in the 90s, with 100s potentially not too far off.

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