Synopsis:
Several major players are affecting our weather this morning as a complex forecast scenario unfolds.
Upper level analysis reveals zonal flow in the upper levels over the northcentral Gulf coast as a jet max associated with the subtropical jet slides just to our south this morning. Further north, weak ridging exists as the flow splits either side of a vertically stacked closed low located just off the eastern VA/NC coast. As you approach the surface, ridging becomes evident in the 700/850mb level. This picture is reflected well in WV imagery this morning which also gives away a larger upper level disturbance pushing eastward across the central CONUS. A rather weak sfc low and associated frontal boundaries can also be found with the warm front/associated upper level divergence responsible for a trail of showers and storms across CNTRL MO/AR.
Closer to home a bone dry airmass is in place with 0z LIX sounding showing a PWAT value of 0.29in. and a noticable subsidence inversion."
"Forecast Discussion (Monday through Friday):
First and second periods will be most affected by the afforementioned upper level wave. Trof axis is forecast by both WRF/GFS to push through around 0z Tuesday. Fcst soundings indicate some disagreement between those two models with GFS being much more aggressive with column moisture. Both models appear to have initialized reasonably well, but NAM has performed better through the first 12 hours of the model run, so will go with the slightly drier NAM and keep all pcpn out of the forecast tonight. This is supported with a lack of overall divergence aloft, and associated low level vertical motion, and an overall lack of a substantial isentropic upglide signal in guidance. Will increase cloud cover slightly to announce the shortwaves presence. For first period high will go with majority MOS which seems to handle the less CAA oriented airmass but slightly lower overall column height well, and go with a high of 68. Will go 1 degree above MOS majority at 46 for second period low.
Tuesday business begins to pick up as by midday a substantial airmass modification is forecast to be underway. We will likely start the day relatively cool and dry but the column is forecast to rapidly moisten ahead of an approaching upper level wave. Main fcst challenge this period is timing of warmfrontal boundary and type/amount of convection/precip that will be associated with it. For now will handle the forecast by increasing cloud cover to Partly Sunny for the Wednesday period and allowing a fairly large wind spread from 5-15 mph from the south as we will start off fairly weak and pick up by late afternoon. GFS/NAM forecast soundings reveal a rather dry airmass aloft with a very thin low level moisture level until later in the evening; so most cloud cover is expected to be in the thin low level strat deck variety. Will go with a high of 69 on Tuesday as WAA should overcome some of the lack of insolation expected from the cloud cover.
Tuesday night is when the forecast becomes the most tricky. Main dynamics move into place as the sfc warmfrontal boundary surges northward as the main upper level trough rapidly and dramatically amplifies. Sfc low responds in kind as do our low level windfields, as well as our deeper layer windfields. Will increase winds to 15-25 mph and introduce a 50% pop with increasing clouds. Severe weather also appears to become possible in the overnight hours Wednesday as instability increases with moisture/warm air advection and speed and directional shear improve dramatically. The best directional shear appears as one would expect to occur while we remain in the vicinity of the warm front in the overnight hours Tuesday. 0-3km SRH values per NAM are at 498 m2/s2 while GFS is somewhat lower at 317 m2/s2. This directional shear however becomes much more unidirectional as the main upper level wave approaches and the flow on the whole becomes stronger, which is after the warm front moves well to the north. That will present the question as to whether or not we will become unstable enough and have sufficient shear in place for much in the way of convection/severe weather and subsequently, isolated tornadoes that would be associated with such a scenario. At this point it does not appear that scenario is particuarly likely but later shifts will need to keep a very close eye on this possibility.
At 12z Wednesday GFS projects a very dynamically impressive scenario with the WRF not lagging far behind. GFS paints 20-25kt sfc winds at a 50-60kt LLJ with 700-500mb winds in the 80-100kt category. With this type of windfield substantial moisture advection will be underway and our column is forecast to become rather saturated, which is expected to lead to rather substantial cloud cover which is projected to limit instability somewhat. With GFS/NAM overall picture painting CAPE values around 600-1000 j/kg with LIs in the -2/-3 range at the peak. FROPA timing is the main challenge this morning as ever so slightly slower guidance continues to trickle in, with the best guess now at a 20-21z FROPA Wednesday, but the trend indicates a slightly slower scenario. Agree very much with SPCs thinking on this event which involves the possibility of a few isolated storms/supercells out ahead of the main front in the warm sector due to the more than substantial deep layer shear, with a strongly forced squall line/MCS racing eastward ahead of the front. Timing of this feature is obviously virtually impossible at the moment and that feature will have a major impact on the type of weather we see. If the system is somewhat slower to initiate and doesn't push through til 2-3pm Wednesday we obviously would have a little better instability in place. Using the trof axis to time where the strongest upper level forcing will be in place it appears this is a plausible scenario. For now will go with a 70% pop but hold off on ""severe"" wording in the actual forecast product. In the worst case scenario it appears we could see a rather serious squall line/MCS event with damaging winds possibly large hail in the strongest cells and embedded tornadoes. With the main limiting factor being a bit of a lack in instability, which likely would be overcome by the tremendous dynamics in place. However, it is still too early to pin down the likelihood of such a scenario occuring.
Wednesday evening is only made challenging by the GFS appearing to flatten the front out along the coast and leave us with the possibility of a post 0z FROPA. Given the tremendous dynamics pushing this frontal boundary and the fact it is nearly perpendicular throughout the event to the mean flow have discounted this solution and gone with the more progressive NAM. However, have left a 30% pop in to allow for any lingering postfrontal moisture as both models indicate at the possibility of a little wraparound moisture behind this very powerful deep layer low that may work its way in to our airmass and fall as anafrontal precip. This also allows us to gently slow FROPA if that becomes prudent.
Thursday things calm substantially under strong northwesterly flow at the sfc and aloft. Will keep winds fairly brisk in the 5-15 mph range through Thursday and Thursday Night. Will go slightly below MOS due to the depth and bredth of this apparent plunge of cool air. Friday also looks cool and mostly clear with Friday night being the coolest night of the period as a sfc ridge axis moves across the region and our winds calm.
|
Feedburner
0 Comments:
Post a Comment
<< Home