July 7 CFD




July 7

Strong upper ridge slowly phases east during the period, bringing height falls over NW Alberta, including the elbow region. North Pacific upper low kicks off a potent shortwave trough, which rotates through SW flow aloft over BC and arrives along the continental divide by late in the day. At the surface, hot conditions prevail across the south, with the surface ridge bisecting the province weakening as lee troughing begins to develop along the front range of the Rockies – where a lee low is spawned in the Elbow or extreme NE BC by evening.

The approaching shortwave trough will help to advect an elevated mixed layer plume out of the US intermountain west into the Alberta elbow by late afternoon, steepening mid-level lapse rates. At the same time, relatively rich low level moisture over central Alberta as of Thursday evening will be advected into the central and northern foothills and into extreme NE BC, pooling into the terrain. The combination of steepening mid-level lapse rates atop a moistening boundary layer and daytime heating will lead to MLCAPE values of 2000-2500J/kg near the foothills south of Grande Prairie. This environment remains capped while awaiting an area of upper divergence associated with DCVA on the nose of the upper trough’s mid-level jet max, and should be released by 00Z. 0-6km bulk shear values also increase to 30-35 knots as the jet max arrives, with a broken line of strong multicells and embedded transient supercells pushing NE off the terrain throughout the evening hours. Hail up to golfball-size and the severe wind gusts will be the main threats in the evening, especially in an area bounded by Dawson Creek, Grande Prairie, and Grande Cache, southwestward toward the foothills and into NE BC. These storms should weaken as they persist over NW Alberta into the overnight hours.

Further south along the foothills, any severe surface-based storms should remain confined to the foothills where low level convergence is maximized, and not much further south than the Robb area. Forcing associated with the ridge-riding shortwave trough will be progressively weaker toward the southeast, however it should help to support a few elevated thunderstorms during the late evening as far south as Calgary, which should push east onto the Alberta plain and weaken into the overnight hours.

Comments

Post a Comment

Popular posts from this blog

May 4 - Convective Forecast Discussion

Image
Scattered thunderstorms are expected to develop over parts of southern Alberta on Friday. Westerly surface flow as of Thursday evening is resulting in a dry air mass over southern Alberta, with dewpoints currently just creeping above the freezing mark as of 01Z in the southwest corner - at a slower rate of return than currently progged. Nonetheless, NWP reveals that a push of Pacific moisture is expected to increase low level moisture in a shallow layer, with surface dewpoints approaching the mid-single digits across much of the southern half of the province by tomorrow afternoon. Weak surface troughing will develop from the SE corner northwestwards toward the northern foothills, resulting in a confluent axis that will help focus moisture and lift along a relatively narrow zone. A plume of moist, mid and upper level westerly flow results in morning cloud atop a standing wave over the southern foothills that overspreads the plains to the east until around midday, after which i
Read more

August 13 CFD

Image
August 13 Height falls associated with cooling aloft overspreads much of Alberta on Sunday, as an elongating upper trough of increasing negative tilt digs eastward across BC. Southerly and even southeasterly mid-level flow about the base of the trough will begin to rotate into the province as the upper ridge moves off to the east. The surface pressure field initially responds with deepening lee troughing and the development of a surface low over central sections of the province during the afternoon. Some moisture convergence occurs into the trough and in the vicinity of the developing low, with added terrain effects enhancing pooling along the northern foothills and Swan Hills areas – however, southeasterly flow will not be laden with rich moisture as it originates from drier locales. Therefore, despite progged dewpoints into the low and even mid teens in parts of central Alberta, there is low confidence it will be of any significant depth – due also in part to its brief reside
Read more

July 27 CFD

Image
July 27 ***A risk of severe thunderstorms will exist over much of westcentral Alberta on Thursday, with significant severe hail and damaging wind gusts possible in the late afternoon and early evening*** Discussion: Moderate to strong SW flow aloft will exist over the province on Thursday, with subtle height falls ahead of a somewhat weak shortwave trough that passes through during the evening. At the surface, a weak lead wave that tracks over northern Alberta brings a weak cold front to the Elbow that slides southeast and stalls along the central foothills in the afternoon. Further to the south, a lee low develops northwest of Calgary, with hot, dry downslope flow across southern Alberta. As of Wednesday evening, a pool of mid-teen dewpoints exists over parts of westcentral and northcentral Alberta, with the 00Z WSE RAOB revealing somewhat of a skin layer of richer moisture – however, the depth of low level moisture sampled overall isn’t insignificant. The associat
Read more