The morning run of the NWS NAM forecast model this morning is very interesting, and it appears that an interesting forecast situation is developing over the Tennessee Valley for Friday night and Saturday morning (March 22nd and 23rd). The 72-hour forecast for 500 mb is shown above, valid 12 UTC 23 March. Not much exciting there, with strongest vorticity advection over Colorado and Nebraska and a ridge from the Gulf of Mexico to the Great Lakes. However, the same forecast for 700 mb (below) is very interesting, with an area of strong upward motion forecast to be centered on Tennessee.
The forecast for 850 mb (above) indicates a region of strong warm advection from northern Arkansas eastward across Tennessee. Forecast precipitation for the 3-hours ending at 12 UTC on the 23rd (below) indicates maximum precipitation centered near Nashville.
Finally the NAM forecast of simulated radar echoes (above) indicates an MCS over Tennessee, with strongest convective storms again near Nashville. So, the model forecasts a distinct, nocturnal and early morning MCS, with the vertical motion maximum produced both by meso-convection and low-level warm advection. Back in the late 1970s and 1980s, when the NWS was working hard to improve forecasts of heavy rain and flash floods, this was the type event that the models of that day could not forecast well. I'll keep an eye on how this situation evolves and report further. I do note that the forecast shown above also forecasts a nearly straight line hodograph, above a cool stable surface layer, with very strong wind speeds - a situation conducive to derechoes, if significant CAPE actually develops.
The situation becomes even more interesting in light of the latest NWS forecast for Nashville:
Note that snow is forecast Thursday night and Friday morning, with no thunderstorms until Saturday night.All-in-all, a very complex forecast, and I doubt that the model will get the details correct 3-days out.
No comments:
Post a Comment