[Thunder Bay Severe Convection Personal Analysis #202501] Can Showery Convection Strike the First Thunder of the Season?

With the time zone adjustment this week, spring is gradually approaching Thunder Bay, and the battle between cold and warm air is becoming increasingly intense.

Overview:

Affected by the warm front of a cyclone, convective rainfall is expected in parts of Thunder Bay this afternoon (Friday the 14th), with isolated weak thunderstorms or small ice pellets possible. Maximum hourly rainfall intensity will be 3-7 mm/h, and precipitation will last no more than 1 hour. Since the area of strong convection is very localized and highly uncertain, the possibility of hail cannot be ruled out. It is recommended to carry an umbrella at all times when going out today.

Analysis:

On the night of the 13th, east of the Rockies in the United States, a Colorado Low with an unusually large circulation was rapidly moving eastward and developing into an extratropical cyclone. Today, its vast eastern warm sector will continuously transport moisture from the Gulf of Mexico (American Gulf) to northern Ontario. According to numerical simulations, around noon on the 14th, the southerly jet axis of this huge warm sector will pass near Thunder Bay. The low-level (850hPa/about 1500m) jet core wind speed can reach 25 m/s, and Thunder Bay is exactly located in the jet exit region (where horizontal wind speed decreases), which is favorable for triggering vertical upward motion and convection.

During this convective process, the near-surface is still occupied by cold air (5°C), and the upward motion exists only in the higher region above the inversion layer. This is truly warm-front elevated convection. Due to moisture conditions, precipitation from elevated convection is usually less than that from warm-sector convection (similar to summer thermal convection). It is expected that the total column PW (Precipitable Water) over Thunder Bay will not exceed 20 mm, so we believe that even if hit by strong echoes, the 1-hour precipitation cannot exceed 10 mm. Looking at the thermodynamic conditions before convection (MUCAPE 400-700) and dynamic conditions (low-level vertical wind shear 100-200), they are fairly good, favoring the formation of convective cells and a low degree of organization. However, since the overall environment is relatively dry, we expect convection to develop in the form of scattered cells, with very localized precipitation areas and short duration. In addition, since the near-surface temperature is close to 0°C, the possibility of small ice pellets in stronger precipitation areas due to precipitation cooling cannot be ruled out.

(Figure 6: Today, due to the obscuration by the cyclone's outer cloud bands, we only saw the initial partial phase of the total lunar eclipse )