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| GFS Forecast 500-mb temperature at 1800 UTC 26 Aug 2014 |
With the pocket of cold air, monsoon moisture, and surface heating, thunderstorms are likely across much of Utah this afternoon and are even possible overnight thanks to the dynamical forcing of the upper-level trough (you may have heard some last night). When it comes to storms like these, however, their development and evolution occurs over periods of at most a couple of hours. Therefore, forecasting the where and when is pretty much impossible until you see 'em on radar and even then the skill of predicting location and intensity declines rapidly after 30 minutes. Sometimes the presence of an upper-level trough helps as it can organize the convection, but in this case, the trough is fairly weak and thus it's evolution also has low predictability.
Perhaps in the future we'll have pinpoint forecasts of thunderstorms at long lead times. A new high-resolution forecast model known as the High Resolution Rapid Refresh (HRRR) may provide modest advancement and is featured in today's forecast discussion from the National Weather Service, although you get the gist of the uncertainty from the initial statements.
VARIOUS OVERNIGHT MODELS...12Z NAM...AND 12/13Z HRRR OFFER A WIDE VARIETY OF FORECASTS ON EXACT LOCATION OF TODAY`S PRECIPITATION...SO CONFIDENCE IS NOT HIGH ON DETAILS...BUT GENERAL IDEA IN HRRR SEEMS REASONABLE...AND THIS GENERAL IDEA IS FOR CONVECTION TO DEVELOP BENEATH THE JET FROM ABOUT KANAB TO LAKE POWELL AND THEN QUICKLY EXTEND NORTHWARD THIS AFTERNOON AND EVENING...WHILE ANOTHER AREA OF CONVECTION DEVELOPS IN THE VICINITY OF THE UPPER LOW OVER THE MOUNTAINS OF EAST CENTRAL NEVADA...AND THIS AREA MOVES INTO THE WESTERN UTAH DESERTS LATE THIS AFTERNOON. SOME OF THIS DESERT CONVECTION COULD EXTEND INTO THE WASATCH FRONT THIS EVENING.The last I checked, the hRR was to become operational in late September. I'm not sure if that schedule will hold. I don't expect it will be a panacea for predicting convection of the type we have today, but I'm hoping it will be useful for forecasting mountain precipitation at short (≤18 hours) lead times this winter.
Observationally, there are a couple of factors which seem to offer at least some predictability for convective patterns in mountainous regions such as the Intermountain West. First, there are some fairly consistent "hot spots" for the initial development of deep convection (typically during the early afternoon), and these are not necessarily obvious, that is, they are often not the largest or tallest mountain ranges, etc. Later in the diurnal convective cycle (typically in the evening or overnight), there seems to be a strong tendency for convective activity to favor areas that were unaffected by earlier convection, and thus still have a relatively warm surface and near-surface boundary layer. It will be interesting to see how well the higher-resolution models can get a handle on factors such as these.
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