25 Sept KPNS forecast

posted Sep 25, 2012, 1:59 PM by Michael Baldwin   [ updated Sep 25, 2012, 1:59 PM ]
much like yesterday, the obs show an anticyclone at the surface to the north/east of KPNS, with the ridge axis perhaps right over the western FL panhandle at the moment.  Low dewpoints have remained in the vicinity throughout the day, with the Purdue WRF frontal diagnostic and SPC mesoanalysis indicating a strong theta-e gradient just to the west of KPNS, with the min theta-e and dewpoint right over the location.  A few high clouds can be seen in the visible satellite, with most of the shallow cu located over the ocean, to the east in the FL peninsula, and to the west in the moister air over LA.  

The anticyclone has been moving to the east and is predicted to continue to do so tonight, which should pull the zone of strong moisture gradient across the area overnight.  Dewpoints are predicted by the NAM and WRF to increase into the low/mid 60's tonight, which should result in a warmer min temp than was observed this morning.  I think this is a bit of a tricky forecast, given clear skies and light winds tonight they should quickly set up an inversion just above the surface, that stable air will be relatively difficult to move, so I expect the moisture to return quickly above the surface while the dewpoints at the surface could remain relatively dry until early tomorrow morning after the stable layer is eroded with the morning warmup and vertical mixing quickly changes the surface layer moisture.  Guidance numbers are in the mid-upper 60's range, with MOS values of 67 or 68 and USL predicting a low of 70.  The NWS agrees and currently has a low of 68 there tonight in the point forecast matrices.  I might be overthinking this, but am leaning towards the cooler side of the guidance right now.

Once the sun starts to warm things up tomorrow, the moisture should quickly bounce back.  We should expect to see some cu developing during the afternoon.  Forecast soundings show CAPE values reaching over 1000 J/kg, however, the atmosphere is very dry above the boundary layer, making it difficult for those cu to deepen enough to begin producing precipitation.  Both the NAM and GFS indicate some precip along/near the coast tomorrow afternoon, with the moisture and instability clearly in place, we cannot rule out an airmass thunderstorm later in the afternoon.  With the dry air aloft and basically zero forcing for lift, I am leaning towards another dry day, which is basically what the guidance is telling us as well with MOS and NWS POPs around 10%.  NAM forecast soundings show the classic "shallow convection" signature from the BMJ convective scheme, which says that the model is trying to produce convective precip but cannot since the cloud layer is too dry.  The GFS scheme is active whenever the CAPE increases quickly, which is consistent with a warm/moist afternoon boundary layer.

Temperatures aloft (~700mb) in the NAM Bufkit soundings barely change tomorrow compared with today.  The shallow convection scheme is messing with the layer between the top of the boundary layer and where it thinks the cloud tops will be.  The cu formation will likely limit the max temperatures tomorrow, but with little change to the overall temps, a "persistence" forecast will probably be another good one for the max temp tomorrow.  Their warmest ob so far today was 83, the guidance is clustered around 85-86, I don't have any reason to disagree with those numbers.

Winds look to be quite similar to today, while there is a chance they could get a bit higher winds if one of those airmass thunderstorms ends up affecting the airport, I don't think that is a high enough probability to affect my forecast.  Sea breeze will likely be the main controlling factor on the max wind speed with an otherwise weak pressure gradient.  Guidance numbers are in the 8-10 kt range, the predicted winds at 850 tomorrow afternoon are a few kts higher than today, so going on the higher side of that range sounds good to me.

Again, I expect to see a sharp distribution in all of the forecast values, close to the current NWS numbers of 85/68/10/0.00.  Scoring strategy-wise, it's another day to stay close to the consensus since a couple of points will make a large difference in your normalized score. 
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