Twilight upon the mountains
Joel 2:2 describes “A day of darkness and of gloominess, a day of clouds and of thick darkness, as the morning spread upon the mountains.” The phrase “as the morning spread upon the mountains” may allude to things that hide or obscure the true meaning of the mountains of prophecy. The word translated “morning” is shachar, meaning dawn, or the pre-dawn gloominess.
The Hebrew is כְּשַׁ֖חַר, “as the dawn.” In Hosea 6:3 it is translated “as the morning.” Two meanings are listed in Strong’s Hebrew Dictionary.
 shachar, shaw-khar’, a primitive root; properly, to dawn, i.e. (figuratively) be (up) early at any task (with the implication of earnestness); by extension, to search for (with painstaking):–(do something) betimes, enquire early, rise (seek) betimes, seek diligently) early, in the morning).
F. Meyrick wrote: 
as the morning spread upon the mountains] This is usually taken only to designate the wide extent of the locust bands, far stretching as the morning light breaking over the hills. See Pococke and Chandler, in loc. As however the word used for “morning” is not baker, which is the proper word for “morning,” but shachar (derived from shâchar, which in one of its significations means “to be dark”), it is better to understand by it the twilight or dusk which precedes the full brightness of the day, connecting it closely with the darkness and gloominess just described, and translating “like the glimmering twilight of the morning,” such as it is when it is as yet “spread upon the mountains” only, and has not descended into the valleys. Other explanations have been given. Schmoller, supplying the word “comes,” translates “as the morning dawn spread on the mountains, comes a people great and strong.”
S. L. Warren wrote in Ellicott’s commentary: 
(2) The morning spread upon the mountains.—The Hebrew word here used for morning is derived from a verb, Shachar, which has for one meaning “to be or become black,” for the second “to break forth” as light. From this latter signification is derived the word for morning—dawn; from the former comes the word “blackness,” which gives the name Sihor to the Nile (Isa, xxiii. 3). It seems accordingly more in harmony with the present context to take the sense of the word in its reference to blackness, and to understand it as indicating a thick, dark, rolling cloud settled upon the mountain top. The description following comprehends equally the natural and political locusts.
The idea expressed by the word translated morning in Joel 2:2 seems close to Zechariah 14:6-7, a day that is “not be clear, nor dark,” but something in between. In Joel, it is gloominess associated with “thick darkness,” and clouds, that obscure the mountains. These are no doubt prophetic mountains, not literal ones. Note that the members of the army are said to “leap upon the tops of the mountains” in Joel 2:5. They are pictured in battle array, and much noise is generated by them.
This all seems to relate to the interpretation of the mountains of prophecy. There is a parallel to the description of the invading armies of Gog and Magog in Ezekiel 38:8, who come against “the mountains of Israel.” The mountains are symbolic of the promises and blessings, that are the inheritance of the saints; see Mountains and the promises of the gospel, and Interpreting the mountains of prophecy.
Much noise is generated by prophetic interpreters who claim that Ezekiel’s Gog and Magog prophecy is all about an attack by several nations including Russia and Iran, perhaps a nuclear one, according to some, upon the Jewish state, but they misunderstand; the prophecy was interpreted by apostle John as an assault on the “camp of the saints” and the “beloved city,” which can only mean the Christian church.[Revelation 20:8-9]
The armies described in Ezekiel’s prophecy come against the mountains, as they misinterpret the prophecies of Scripture, and the blessings and promises that mountains represent. The great variety of interpretations is evidence of it. Ezekiel said they come against the “prophets of Israel,” which would rule out a military application of the prophecy, as no military invasion of the modern Jewish state could be construed as an assault upon the prophets of Israel. Ezekiel wrote: “Thus saith the Lord GOD; Art thou he of whom I have spoken in old time by my servants the prophets of Israel, which prophesied in those days many years that I would bring thee against them?” [Ezekiel 38:17] This can only refer to some kind of spiritual conflict, such as the “spiritual wickedness in high places” that the saints wrestle against in Ephesians 6:12, which would probably include discussions about the interpretation of Scripture.
The armies of Gog and Magog come against “the land of unwalled villages,” but that does not apply to the land of the Jews in Palestine, where the Israeli-West Bank Separation Barrier is a prominent wall consisting of concrete slabs 8 m high, and fences of barbed wire, that stretches for about 760 km. Thus, Ezekiel’s prophecy cannot be applied to the existing Jewish state. On the other hand, the flawed interpretations of prophecy fulfill Ezekiel’s prophecy; and these are also a source of the gloominess on the “mountains” of Joel’s prophecy, as they tend to obscure the symbolic meaning of the prophetic mountains. What the prophets meant as good news, and blessings for the saints, dispensationalists interpret in a depressing, gloomy, pessimistic fashion; either the mountains are the literal mountains of Palestine, or else heathen nations, or obstacles, problems, etc.
Another connected prophecy is found in Revelation 16:20, where “the mountains were not found.” The mountains here are symbolic; the prophecy means the promises to the saints are not found, that is the spiritual blessings that are intended for them.
Jacob described the blessings and promises he received as extending to “the utmost bound of the everlasting hills,” when he blessed Joseph. [Genesis 49:26] And among the blessings assigned to the tribe of Joseph in Deuteronomy 33:15 were “the chief things of the ancient mountains,” and “the precious things of the lasting hills,” which are none other than the promises to Abraham, Isaac and Jacob, early expressions of the gospel.
1. F. Meyrick. Joel. In: Frederic Charles Cook. The Holy Bible, According to the Authorized Version (A.D. 1611): Ezekiel. Daniel, and the minor prophets. J. Murray, 1876. p. 504.
2. S. L. Warren. Joel. In: An Old Testament commentary for English readers, by various writers, edited by Charles John Ellicott. 1884. p. 441.