The morning balloon launch from Tallahassee came back with a mostly dry atmosphere, but shallow ground fog may persist for another few minutes in swampy areas or near bodies of water. Moisture channel imagery and upper air observations depict a vigorous shortwave trough translating eastward across the middle Mississippi Valley. This disturbance will stay well to our north today, but sprinkles or a brief shower could make it as far south as Atlanta, Birmingham, or Raleigh. The bulk of any serious cold air also remains well north of us, so highs will be in the mid 70s today and Monday. The deep column of dry air and nearly calm winds will permit temperatures to drop into the upper 30s in the outlying areas Monday morning, with urban areas and the coast staying in the mid to upper 40s. As was the case this morning, a shallow layer of fog will probably form around sunrise near any bodies of water or excessively saturated ground from the recent rains.
A very powerful deep-layer cyclone is about ready to slam into California. It is already seeing heavy mountain snows and valley rains. The numerical models agree on weakening the trough somewhat as it moves across the Rockies, but it is forecast to reamplify as it reaches the Eastern United States on Tuesday. As I mentioned yesterday, strong vertical wind shear (50 knots or greater in the lowest 6 kilometers of the atmosphere) will be in place ahead of this trough and our area will be favorably positioned with respect to the middle and upper-level jet axis. The latest deterministic and ensemble models now forecast at least modest instability in response to a low-level southwesterly wind flow that should act to advect lower to mid 60s dewpoints back into our area. Despite the increasingly favorable thermodynamic and kinematic environment predicted by the models, the deep-layer wind flow will be veered because the trough will have a positive tilt. This will limit convergence and could reduce the convective coverage. There is also evidence that a capping inversion (or stable layer) will be present. However, if sufficient convergence can overcome the capping, strong to severe storms will develop. The low-level and deep-layer shear vectors will be approximately parallel to each other, so thunderstorms will likely organize into several small lines or clusters with the potential of damaging wind. Mid-level lapse rates may also favor marginally severe hail with the strongest updrafts. Since the thunderstorm threat remains conditional, the Storm Prediction Center has introduced just low probabilities of severe weather for now.
Unlike the last several storms, the airmass behind this one will be much colder. A deepening northerly flow will send us some mighty chilly air from Canada. Temperatures will probably stay in the 50s on Wednesday and perhaps Thursday. The models are in remarkable agreement in showing the center of high pressure parking nearly right on top of us Thursday morning. If this verifies, a hard freeze will occur. I am forecasting a low of 26 degrees, but a couple of the ensemble members show the low temperature getting as cold as 23 degrees. If the model guidance keeps predicting the same pattern this afternoon, I will get more aggressive in forecasting this hard freeze. Stay tuned for additional details on this in the coming days.
The pattern is expected to at least calm down, with no major storms expected between Wednesday and next weekend.